This patch does the following :
- as a vout window provider, make sure skins functions run by vout thread(s)
  don't issue any calls to the GUI. (skins on Linux runs fine at last !)
- prepare support for SET_VOUT_SET_FULLSCREEN new control
- implement hotkeys (no longer available at the vout level)
- modify a show() method not consistent across platform
- cleanup (dead code)