MapBaseXML.txt is not a gutted .tmx anymore, it only has a list of layers.

MapBaseXML.txt used to be a .tmx file with all the desired tilesets plus
the layers (without any tile layer data).  When BMPtoTMX ran, it just filled
in the tile layer data as a comma-separated string.

One big problem with this was that the path to each tileset image was
hard-coded as ../Tiles/sometileset.png.  The user is permitted to generate
BMPtoTMX .tmx files in *any* directory, so tilesets might not be found when
opening the .tmx in TileZed.  WorldEd would find the tilesets by magic if
the Tiles directory happened to be in its list of search paths.

Another problem was how the user could add new tilesets to be included in
the generated .tmx files.

The solution is to have MapBaseXML.txt just have a list of layers that should
be in generated .tmx files.  All the tilesets are taken from Tilesets.txt.
The Tiles directory is retrieved from TileZed's settings BUT that only works
when those settings are in the Windows registry.  Gotta fix this on Mac/Unix.

A related change: the generate-lots dialog no longer requires the user to
specify the Tilesets.txt file  TileMetaInfoMgr::setTilesDirectory made
no sense, since it was specifying the directory containing Tilesets.txt and
not the Tiles directory containing the tilesets.  The location of Tilesets.txt
is assumed to be ~/.TileZed.
Fixed 2 cases where BMPtoTMX creating new maps didn't update the display.

If a cell had a map assigned but that map didn't exist when the world was
loaded, then:
1) No world-scene image would be displayed (unless the image existed
   for some reason already), and
2) An empty placeholder map would be displayed in a cell-scene.

If the map was then created by BMPtoTMX, the world-scene image would not be
created and the cell-scene would not replace the placeholder map with the
new map.
Added Tiles Directory to the preferences, removed search paths.

WorldEd had a list of directories to search for tilesets and maps, which is
ass-faced.  It makes to sense to "go looking" for tilesets or maps when their
locations should be exactly specified or relative to some known file or directory.
This behavior was only added to deal with the fact that TileZed did not save
the location of lot maps, it only saved the file name (without extension either).
Now that TileZed was fixed to save the location of lot maps this list of search
path is defunct.
Fixed the window title of the preferences dialog.
Fixed BMPToTMX memory leak with images.

I should free the images after each call to generateWorld, but only do it
on the next call.
Fixed BMPToTMX not clearing the list of layers or conversions map.
Plugged a massive memory leak with the 'Generate Lots' command.

When calling QVector::fill() the compiler-generated Square::operator=() was
getting called and it didn't free the Square::Entries list.

I could eliminate the Entry class and just keep a list of LotFile::Tile* instead.