Implicit meta-enums used for W and N tiles.
Ensure .bmp images are 32-bit ARGB for fast pixel manipulation.

Lemmy had an 8-bit image which was taking forever to create a thumbnail for.
Fixed crash when a displayed map was changed on disk.

The "Not safe enough" code should have been done even when the top-level
map was the one that changed, but was only being done when a submap changed.
Cell map's 0_Floor tile is kept if a submap does not have a tile in that layer.

After a chat with lemmy, it was decided that if a lot map doesn't have a tile in
the 0_Floor layer (such as along the outside wall of a building) then the cell's
0_Floor tile should be used.

Normally the presence of *any* tile in a submap causes all other tiles at
that location to be replaced with the submap's own.
Copied the more-efficient TileLayer::erase() method from TileZed.
BMPtoTMX warns about unknown colors used in images.

The warning can be disabled via a checkbox in the BMPtoTMX dialog.
Also fixed assignMapsToWorld not being saved in .pzw files.
BMPtoTMX works with various image formats now, including .png.
Added file-path tooltips to BaseCellItem and WorldBMPItem.
Added reference-counting to MapManager to curb memory usage.

WorldEd never freed a map once it was loaded. This was a problem when generating
many thumbnails or generating .lot files.  All but the 50 most-recently-referenced
maps are purged whenever a map is dereferenced.
Used TilesetManager tileset reference counting on MapManager's maps.

Since maps are now freed, it is important to free a map's tilesets as well.
Fixed LotFilesManager not handling sub-map tilesets recursively.

This was only an issue when a lot map had its own lot maps.
Ported better MapManager::convertOrientation from TileZed.
Ported Tileset::clone from TileZed.