Added experimental change which allow to use alt-tab and other shortcuts on linux when game is run in fullscreen mode.

This method is used in many other projects and should work good. Currently modern games
use window manager fullscreen rather than grab whole mouse and keyboard.

Tested on Gnome, Unity and Openbox and it works well. Though it should be tested also on
other desktop environments.

Also performance impact should be tested.

I added simple workaround for Gnome, which sometimes creates fullscreen window smaller
than display.

If some problems will occur, feel free to revert it.
Merge branch 'master' of https://github.com/supertuxkart/stk-code