Allow .tbx files to be used as Lots.
Fixed .tbx safe-saving breaking the FileSystemWatcher system.

When a .tbx file is saved, the original file is first renamed to *.bak then
the new file is written.  The QFileSystemWatcher class reports the renaming
of the original file being watched but then stops reporting any change to
the new file.
Ported some code from TileZed that fixes an issue with changing a tileset's imageSource().
Don't ask TileMetaInfoMgr to load TilesetManager's missing tileset.

BuildingMap adds the tileset manager's "missing" tileset to a map.  That was
needed so the lot generator detects the missing tileset if it is used.
Load map thumbnail images in a separate thread.

When a map thumbnail is requested, a dummy image is returned (just a gray
polygon the size of the map) and the actual image is read in by a worker
thread.  The thread sends a signal when the image has been read, at which
point the display is updated.