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.
Added mouse-over highlighting of Lots that are part of a cell's .tmx file.

Lots added in WorldEd have a scene object to make it obvious they can be
edited.  But Lots that are part of a .tmx don't have a scene object.  Because
all Lots may be right-clicked for "Open in TileZed" action, I added a mouse-over
highlight to .tmx lots when using SubMapTool.
Fixed MapManager getting recreated during application exit.
Lemmy request: draw RoomDef objects with bogus names in red.
Added command for mass remapping of tiles from one group to another.

This command is for replacing tiles in existing TMX files.
Tiles that are in a group will get replaced with a random tile in another
group.
Groups are specified in a SimpleFile.
Improved reporting of unknown colors in BMPToTMX images.

Now there is a list of up to 50 positions for each unknown color.
Also fixed reporting world coordinates rather than image coordinates.
Really stop MapManager processing maps recursively.

When a thread finishes loading a map, it sends a signal which is delivered
via the application's event loop.  One problem with this is it can happen
any time the application processes events.  Sometimes the progress dialog
is put up while waiting for a map to load, while calling qApp->processEvents()
to keep the app responsive.
Fiddled with LotPackScene's highlighting of suspect RoomDef names.