Critical bugfix and other things

The tile combination code was flawed I'm so sorry, I hope nobody saved a room in that version.

This will hopefully be the last release in a while.

Other changes
-Added option to add a new layer with depth to the layer dock
-Made the tileset import plugin handle pngs
-Loaded templates will now emit the changed signal after generating templates, this fixes some graphical errors
caused by outdated image indices.
-Now a dialog will popup when you load a background, it will auto complete the tile size to that of the last room loaded
or 16,16 by default. There's also a checkbox to set the tile size from the backgroundfile, but that's not a good idea,
because things work better in general when the tilesize of the tileset matches that of the map.
-The room import dialog now remembers the last tile and quad sizes you used