Map editor: Make tile selection fully functional

 - Add zoom, grid and offset buttons to the tile selector
 - Redraw the tiles in the selector as needed, including when switching primary
   map layer
 - Reuse same loop code for drawing, selecting and sizing tiles
Map editor: Save tile list width and offset in the project file
Halloween Harry: Add sprite tileset
Add startup message about this being an early preview version
Map editor: small bugfixes
Map editor: Remove currently broken object-mode code until it is fixed