Virtual tilesets can now be used everywhere that non-virtual tilesets can.
Added reading/writing VirtualTilesets.txt.
VirtualTilesetView: only drag already-selected tiles.
Added floor and roof virtual tiles.
Added VirtualTilesets.txt.
Added "shallow slope" and "trim" roof virtual tiles.
VirtualTilesetDialog can show the disk image for a tileset.
AddVirtualTilesetDialog displays cols/rows in tileset disk image if any.
Added "short wall" virtual tiles.
Improvments to AddVirtualTilesetDialog.
Added TileShapeEditor.

A dialog for editing the geometry and u/v coordinates of virtual tile shapes.
Currently does not edit u/v coordinates.