Add TextureMgr, VirtualTilesetMgr etc.
Late commit of HeightMapView changes to how wall tiles are rendered.

This predates the virtual tileset stuff.
Some fixes for GCC.
Fixed WorldCellTool not receiving mouseReleaseEvent() after double-clicking to open a cell.

Seemed to become a problem in Qt 5.
Ported a MapManager change from TileZed.