Implement a fullscreen on Linux

[doublec]

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.

[doublec]

Rebased on top of master.