What I'd like to do some time...

* Fullscreen toggle needs to account for Xinerama.

* New windows tile as well as possible.

* Seems like some applications (notably audiocontrol) bring themselves
  to the front whenever mapped.  This breaks the simple approach to
  keeping stacking order on changing desktop.  So probably have to do
  the structure reordering thing.

* Think of some good way of adding controllable window resizing beyond the
  fullscreen/vertical maximise toggles.
