Improve window handling in overview mode

Improve window handling in overview mode

* For small window counts, lay the windows out according to a
  predefined scheme. For larger window counts, continue putting
  the windows along the diagonal as before, but do it a bit better.
* Special case the desktop window and use it as the background
  of the window area.
* Add a little translucency to windows
* Use tweener to animate everything into place
* Clean up - add constants and break things into multiple methods

svn path=/trunk/; revision=32