THIN BRICK PANEL CONSTRUCTION CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation of U.S. Patent Application Serial No. 09/533,200 filed on March 23, 2000, which claims priority of U.S. Provisional Patent Application Serial No. 60/126,558 filed on March 26, 1999. BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an improved building block holder used to support and position building blocks, such as so called "thin bricks", which are used as surface panels for buildings. Statement of Related Art In traditional building construction, block walls are formed by stacking blocks one at a time on top of each other with a layer of mortar between the blocks. Such construction is typical for brick walls as well as walls formed of other types of building block materials. Such walls are aesthetically pleasing but the materials are expensive and the walls are expensive to fabricate and are poor insulators. A less expensive alternative is a so-called "thin brick" wall construction, wherein thinner bricks are mounted on building block holders in the form of flat support panels that are affixed to the sides of the building. Gaps between the thin bricks are filled with traditional mortar, so ttiat the finished panel has the same visual effect as a traditional brick wall. However, the panels are thinner and are less expensive and easier to install. Such panels also facilitate the incorporation of efficient insulating materials, such as foam board, between the panel and the building walls. One problem with the use of thin bricks is that they can sometimes become separated from the support panels on which they are mounted. The traditional way for affixing thin bricks to support panels is by a conventional adhesive. If the adhesive releases, the bricks can fall off the panels. Some systems have used separately attachable clips that incorporate devices to lock mortar on the panel but these are extra elements, involve extra expense, and provide only a limited nunϊber of attachment locations of limited effectiveness. An object of the present invention is to provide an unproved building block holder for building blocks such as thin bricks or the like, wherein the building blocks are positively locked to a support panel by a plurality of integral, durable mortar locks that lock the mortar to the panel. Another object is to provide mortar locks that not only lock the mortar to the panel, and thus hold the bricks to the panel, but also serve as flanges that support the bricks in horizontal rows.

SUMMARY OF THE INVENTION The present invention comprises an improved building block holder for supporting and positioning thin bricks or other building blocks in a spaced relationship on a wall with a gap between adjacent blocks adapted to be filled with mortar. The building block holder comprises a panel formed of sheet material having a plurality of rows of block support elements. The rows are spaced vertically apart by the height of the blocks and the gap between the blocks. Each block support element has a portion that extends at a nonperpendicular angle with respect to the panel, such that there is void behind that portion of the element. Thus, when the gap between the blocks is filled with mortar, some of it will occupy the void so that when the mortar dries, the mortar and the blocks will be locked on the panel. The block supporting elements of the present invention desirably are formed in the shape of upwardly and"outwardly"extending prongs having vertically positioned reinforcing ribs therein. This gives the prongs a rigid construction that resists deformation during storage or handling of the panels and prevents the prongs from bending downwardly when building blocks are placed thereon. The present invention also incorporates rearwardly deformed portions adjacent the bricks that serve to space the panel away from a substrate and also provide a recessed location for mounting fasteners and a water channel for directing water from behind the panels. The present invention also includes integral glue locks positioned so as to be adjacent a rear portion of the bricks. The glue locks comprise openings in the panels or nonperpendicular projections from the panels such that adhesive used to attach the bricks to the panels can penetrate to a position behind the projections or behind the panel so as to lock the glue on the panel and thereby lock the bricks on the panel. The present invention includes a number of additional accessory features, including a shelf for mounting bricks on their ends, a corner bracket for mounting L-shaped bricks to the corner of a building, and an edge connector in the form of a J-piece for attaching the edge of a panel to a building. These and other features of the present invention are described in detail below and shown in the appended drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a building block holder constructed in accordance with the present invention. Fig. 2 is a plan view of the building block holder of Fig. 1. Fig. 3 is a sectional view taken along lines 3-3 of Fig.2. Fig.4 is a fragmentary side edge sectional view showing the block holder of the present invention in connection with a block wall assembled on the wall of a building. Fig. 5 is a fragmentary perspective view showing the brick support prong and mortar lock of the present invention. Fig. 6 is front elevational view of the prong of Fig. 5. Fig. 7 is an end view of the prong of Fig. 5. Fig. 8 is a fragmentary view of the rear side of the panel showing the glue lock of the present invention. Fig. 9 is a perspective view showing a shelf for mounting bricks on their ends. Fig. 10 is a perspective view showing a corner panel employed in the present invention. Fig. 11 is an top edge view showing a J-shaped edge piece for attaching the edges of a panel to a substrate. Fig. 12 is a side edge sectional view similar to Fig. 4 showing a block holder bonded to a foam board backing. DETAILED DESCRIPTION OF THE INVENTION Referring to the drawings, a building block holder 10 for supporting a plurality of thin building blocks 12 such as blocks known as thin bricks, is shown in Fig. 1. Block holder 10 comprises a flat panel 11 formed from a sheet material suitable for use in building construction. Preferably the material is a sheet metal of the type that resists corrosion. A galvanized sheet metal is desirable. Other materials could be employed. Typically a panel is about 2.4 meters (eight feet) high and about 0.4 meters (sixteen inches) wide. Panel 11 comprises a series of rows of block support elements 14, each spaced horizontally apart along the panel. The block support elements are spaced far enough apart that they provide a multiple point support for bricks 12 placed on the support elements. A typical thin brick might be seven and one half inches long, two and one quarter inches high, and one half inch thick. The brick support elements are conveniently positioned about two and one eighth inches apart in such a construction, so that each brick is supported by multiple brick support elements along its length. The construction of the brick support elements is shown in more detail in Figs. 5-7. Each brick support element 14 comprises an outwardly inclined prong 16 preferably formed by being stamped out of a triangular section of panel 11. This leaves a triangular opening 18 in the panel after the prong has been stamped outwardly therefrom. Prong 16 includes a nonplanar corrugation or rib 20 that extends upwardly along the length of the prong for purpose of reinforcing the rigidity of the prong. This makes the prong less likely to become deformed during storage, handling, or shipping, and also makes the prong less resistant to downward bending when a block is placed thereon. The prong's form thus has the general shape of a spout. The nonperpendicular or angular position of the prong with respect to the panel creates a space 28 behind a rear surface 22 of the prong for receiving mortar 24 (see Fig. 4). The spaces behind the block support elements and the openings in the panel behind the block support elements serve as mortar locks for locking mortar on the panel after mortar has been applied in the gaps 26 between the bricks. Typically, the gap between the bricks is about one half inch wide. When mortar penetrates the space 28 behind the rear surface 22 of the block support elements and when the mortar further penetrates through the panel 11 through the opening 18 and thereafter is permitted to dry, the mortar engages the rear surface 36 of the panel and the rear surface 22 of the block support elements 14 and is thereby locked on the panel 11 by affirmative mechanical action. Since the mortar 24 also engages and locks the bricks 12 in position in the mortar when the mortar hardens, the mortar locks serve to lock the entire sheet of brick and mortar on the panel. Referring now to Figs. 1, 4, and 8, the panel 11 also includes a plurality of glue locks 30 spaced in horizontal rows along the panel at a position just below the brick support elements 14. The glue locks 30 are desirably lance openings 34 shaped so as to provide an arcuate segment 32 extending rearwardly from each opening 34 in the panel 11, as shown in Fig. 8. As shown in Fig. 4, bricks are attached to the panel by applying adhesive 78 or glue to the panel and then resting the bricks 12 on the prongs 16. The glue 78 is applied to the area of the panel 11 covered by the glue locks 30. The glue 78 flows at least partially through the openings 34 in the panel and outwardly adjacent the rear surface 36 of the panel. This causes the glue 78 to become locked to the panel 11 when the glue dries. The glue 78 also penetrates the pores of the bricks 12 and secures the bricks 12 to the panel 11. Another element of the panel 11 is a plurality of longitudinal recesses or grooves 35 in the panel, generally formed by roll forming the panel. The longitudinal recesses 35 serve a number of purposes. First, they serve as spacers to space the panel away from the substrate (See Fig. 4). With the panel 11 spaced away from the substrate, the glue lock segments 32 can extend rearwardly from the panel as shown in Fig 8, and there is a space 80 behind the panel 11 adjacent the glue locks 30 and adjacent the brick support elements 14. Thus, when mortar 24 and glue 78 enter the openings 34 in the glue locks and the openings 18 in the panel 11 adjacent the prongs 16, mortar and glue can flow completely through the panel and at least partially behind the panel in order to provide a more secure lock for the mortar and glue to the panel. The grooves or recesses 35 in the panel 11 also serve to reinforce the panel and to channel liquid flow from the panel. They also serve as recessed locations for inserting fasteners such as screws 37 into the panel to mount the panel to a substrate. With the recessed location, heads of the fasteners 37 can always be recessed below the rear surfaces 22 of the blocks 12 and will not interfere with the positions of the blocks. At the bottom of the panel 11, a flange 38 extends outwardly from a rear surface 39 at right angles. Bricks 12 are placed on the flange 38 and glued to the panel 11 in the same manner as bricks attached elsewhere to the panel. At the top of the panel 11, tabs 40 spaced along the top edge of the panel are deflected outwardly at a forty-five degree angle. These tabs alternate with vertical tabs 42 and tabs 44 that are inclined in an opposite direction along the top edge of the panel. The tabs 40, 44 act as a further spacer from the substrate and also engage and position the lower end of the next adjacent panel above, while at the same time holding the top edge of the panel as reflected in the vertical tabs 42 behind the lower end of the panel above. The panel configuration can be modified at the top of a wall so as to eliminate prongs at the top. Whereas, a normal panel may be two feet high, separate panel sections sixteen and eight inches high can be used at the top of the wall. The sixteen inch section can have prongs on the lower part. A number of accessories for the present system are shown in Figs. 9-11. In Fig. 9, a shelf 43 is employed for bricks 12 that are to be mounted endwise on the panel. The shelf 43 provides a wider, more stable base for endwise bricks 12. Shelf 43 comprises an upright back plate 44 having a groove 46 therein that nests in groove 35 in the panel. A lower perpendicular flange 48 extends outwardly and supports the lower end 50 of an endwise positioned brick. The brick holder 10 of the present invention can also include a corner panel 52 that fits on the corner 54 of a building. Corner panel 52 comprises a section of sheet material formed into a right angle bend, with each side of the bend including a corner brick mounting surface 58 and an offset tab 56 at the end for attaching the corner panel to the building. Surface 58 is spaced away from the building, and lance openings 60 or other suitable openings in the panel serve as glue locks for an L-shaped corner brick 62. A normal panel 11 of the type described above can be fastened on the offset tabs 56, as shown in Fig. 10. The edges of a panel 11 can be fastened to the substrate by means of J-shaped channel members 66, as shown in Fig. 11. Inner leg 68 extends inwardly further than outer leg 70 of the J-shaped member 66, so that a fastener 72 such as a screw or rivet can extend through leg 68 without extending through leg 70. Looking now at Fig. 4, in operation, the panels 11 are first attached to a substrate by screws 37 or the like. The substrate may be studs 74 on the side of a building and may include foam board 76 or other insulation panel on the outside of the building (Fig. 4). Other construction methods or components also could be used. The building blocks 12 are then mounted on individual panel members by a conventional adhesive glue 78. The glue is applied adjacent the glue locks 30 so that the glue 78 penetrates the panel 11 and becomes at least partially locked behind a portion of the panel. After the glue 78 has cured, mortar 24 is applied to the gaps 26 between the bricks 12. The mortar 24 flows behind the prongs 16 that support the bricks 12 and also flows through the openings 18 in the panel behind the prongs and penetrates the space 80 behind the panel itself. When the mortar 24 dries, it thereby becomes locked to the panels 11 and in turn locks the bricks 12 to the panels. The prongs 16 thus serve the dual purpose of supporting and positioning the bricks 12, while serving as mortar lock for locking the mortar 24 and thereby locking the bricks 12 to the panel 11. When a panel 11 relies solely on glue 78 to hold the blocks 12 to the panel, the glue 78 sometimes breaks and releases blocks from the panel. With the mortar 24 serving as an additional locking means for the blocks 12, the blocks are locked permanently on the panels 11. As an alternative to mounting the panels over foam board as separate elements, panels 111 in Fig. 12 are bonded by contact adhesive to a polystyrene foam board backing 76' (which may be 1/2 to 2 inches thick). In panel 11 ', lance openings 30' desirably are the same depth as recessed portions 35' and the foam board is grooved so that the lance openings and recessed portions fit into the foam backing. This maximizes the contact area for the contact cement. The adhesive for the bricks includes a solvent that erodes the foam that it contacts, so the adhesive 5 makes its own opening in the foam to lock the bricks on the panel. In this construction the bonded foam board and panel are installed as one unit on the side of the building. It should be understood that the foregoing embodiments are merely exemplary of the preferred practice of the present invention and that various changes and modifications may be made in the arrangements and details of the construction of the embodiment disclosed herein without departing from the spirit and scope of the present invention, as defined in the appended claims.