Changed Tile::mImage from QPixmap to QImage to allow multithreaded use.
Read maps in threads.  Draw image thumbnails in threads.
TileLayer reference-counts used tilesets.

MapImageManager needs to know (quickly!) which tilesets are used by a map.
Instead of scanning through every tile in a tile layer, keep a dictionary
mapping a tileset to the number of cells using tiles in that tileset.
Added 'const' to an argument where GCC complained (but not MSVC).
Make sure thread-loaded tileset are added to FileSystemWatcher.
Made 2 global functions into methods of FurnitureGroups.

The only reason they were global is because I thought the header would have
had to include simplefile.h, but that isn't so because only references to
SimpleFileBlock are used.
Fixed "return 0" being used instead of "return QString()".

It worked with MSVC but not GCC.
Don't crash or complain if RoofObject is passed 0/none for cap/slope tiles.

Assigning "none" isn't actually allowed, but when reading some old building
files it would crash.
Maps/buildings load in threads, worker-objects, etc.

See TileZed commits at this time for comments.  :-}
Added WorldMiniMapItem.

The world minimap shows all the BMPs in the world.