Porting Gnome Shell 3.8 to use XI2

Mutter and Gnome Shell 3.8 will be port to use latest input X.org extensions XI2! Porting Mutter to XI2 is the prerequisite for enabling multitouch and gesture support in gnome-shell. It is also needed for triggering the message tray by downwards pressure, as it was designed to do. It is uncertain if multitouch and gesture support will arrive in this cycle but the first step is done!