Added automatic layer-switching to BuildingEd's tile-edit mode.

The tileset dock gained a toolbar with a button to toggle the automatic
layer-switching option.

The tileset dock gained a context menu to change the default layer for
selected tiles.
Fixed tile layer names not being read when the image source for a tileset changed.

BuildingEd wasn't displaying the default layers for tiles because TileMetaInfoMgr's
tilesets have relative filenames (and are marked missing) when addReference()
is called, so readTileLayerNames() wasn't finding the tile layer names .xml files.
Added a <None> option to the Default Layer context menu in the tileset dock.

For some reason I didn't provide a way to clear the default layer name
assigned to tiles.
Added WallFurniture2 layer to BuildingEd.

I saw a lightswitch a shelf failing to coexist in the same space at the same time.
Delete the ToolManager and all tools when deleting BuildingEditorWindow.

I got a crash when TMXConfig.txt failed to load, fixing it, then trying to
open BuildingEd again.
User-defined layers in TMXConfig.txt are now added to building maps.

Previously, user-defined layers were only added to exported .tmx files,
they weren't used when editing tiles in BuildingEd.
Removed unused code from buildingtmx sources.
Save/restore the iso/ortho view mode and iso-view scale in BuildingEd.
Fixed the behavior of ReverseLogic.
Fixed BuildingEd's layers view not setting the checked state correctly.
Fixed a crash when creating a new .tiles file.

It crashed when mTileDefFile was nil.

Also changed the default directory to the universal Tiles directory.
Didn't check what happens if that directory isn't set.
Added Help menu and docs to the tile properties editor.
Require Integer properties to have Min < Max.

It used to pass if Min == Max.
Reverse the boldness of checkboxes when a Boolean property has ReverseLogic=true.
Removed the example "modify" block from TileProperties.txt.

All these commands are now documented in the help .html file.