HeightMapScene only shows 3x3 (or less) cells.

Instead of allowing scrolling through the entire world, just show a 3x3 grid
of cells centered on a specific cell.

Also implmented nice wireframe rendering using shaders.
See http://www2.imm.dtu.dk/~jab/Wireframe/.
Implemented undo/redo for heightmap.
Added temp icon for the heightmap tool.
Implemented saving modified heightmap chunks.
Added saving/loading heightmap filename in a world file.
Added undo/redo for setting heightmap filename.
Added mesh and flat heightmap display styles.
'Generate Lots' writes heightmap byte to chunks.
Display cell/lot images in HeightMapView's minimap.
Left a line out from a previous commmit.
Fixed saving when HeightMapDocument was current.
HeightMapDocuments are placed after CellDocuments.
Center HeightMapView on the cell's center when opening.
Playing with lighting shaders (doesn't work).

Need to calculate a fake normal vector for heightmap mesh?
Since the display is orthographic, the triangles are all z=0.