Implement a fullscreen on Linux

This provides a 'toggleFullscreen' method on the xWindowCanvas. Sending
this message will enter or exit fullscreen mode. In fullscreen mode the
window decorations are removed and the window takes up the entire screen
area, covering any window manager panels, etc.

To call it the following suffices:
  desktop w winCanvases first toggleFullscreen

A followup from this would be to find the right place to put 'fullscreen' in the right place for the non-platform dependent backends and implement it for the Mac OS X backend.