Allow priority loading of some maps over others.

Now that adjacent maps are in, loading cells can take some time.
I want the cell maps to show up as quickly as possible while loading of
adjacent maps and lots continues in the background.

So give highest priority to loading cell maps.
Give medium priority to loading adjacent maps.
Give lowest priority to loading Lots in both cell and adjacent maps.
Allow the tiles directory to be chosen for each .tiles file.

Since the .tiles files are stored in the game's media directory, it is
annoying that the tilesets are assumed to be in the same directory as the
.tiles file.  So now you can choose the tiles directory for each .tiles file
via a menu command.

The directory for each .tiles file is stored by QSettings.
Added a tile-picking tool to BuildingEd.

Currently this will select the tile in TileDefDialog so lemmy can edit the
tile properties.

Need to add displaying the tile in BuildingEd's tileset dock.