* all: show/hide the interface with middle-click on the vout

* all: show/hide the interface with middle-click on the vout
       (when the last vout is closed, the interface reappears)
       Works only with the skins interface and X11/Xvideo vouts, but it's
       very easy to implement (just add a callback on "intf-show" in the
       interface, and trigger it in the vout)