Update TileLayersPanel when a layer is renamed.
TileLayersPanel displays hidden layers with a different background color.

Also, clicking in a layer name toggles that layer's visibility.
Updated building *.txt files.
Improved the appearance and behavior of TileLayersPanel.

It used to use MixedTilesetView, now it has a custom class.

The layer name is centered horizontally, and is underlined when the mouse
hovers over it to indicate it can be clicked.

The current layer is surrounded by a black rectangle rather than painting
the current highlight color over it, so it is easy to see whether a layer
is visible nor not (since the bg color is gray when a layer is hidden).

Added mouse-over highlighting.
Used the new MapDocument::setLayerVisible() in several places.

The LayerModel should not be responsible for hiding/showing layers.
Don't show empty () in a tool's tooltip if it has no keyboard shortcut.
Added "ALT = eyedrop" to Door/Window/Stairs BuildingEd tool status text.
Fixed building tile and grime issues in north-west corners of rooms.

Furniture in the wall layer in a NW corner would erase any wall tile there.

Grime wasn't handled properly when a door and/or window was in a NW corner.
This required adding a second layer for wall grime, WallGrime2.

Two Furniture objects forming a NW corner wouldn't keep both tiles.
Fixed sometimes-off-by-one error displaying tiles in the TileLayersPanel.

Made the same change in MixedTilesetView.
Don't reset the TileLayersPanel list when it isn't needed.

This is to work around the issue that selecting the current item scrolls
the list.
Fixed ignored label height in LayersPanelDelegate::sizeHint().

This never happens, there's always a tile.
Removed unused code in buildingfloor.cpp.
Add south-east "cap" tile to the ends of walls.

This handles WallObjects that do not join another wall at the east or south
end.