This patch proposes a fullscreen controller for the skins2 interface.
it features the following :
- skinable fsc (desciption in theme.xml with the Window tag)
- only restriction : no spawning of subwindows or dialogs
- window is considered a fsc when id="fullscreenController"
- (de)activation by clicking on the middle button of the mouse (intf-show)

This fsc currently only works for Linux (new vout design).
For Windows, it is a noop as long as the old vout design is still valid.