GmWindow: Added stay_on_top property.

It is usable with GSettings bindings.
GmWindow: Fixed .h with missing declaration.
GmVideoWidget: Show/hide logo with when the widget is shown/hidden.

Simply connecting to the show signal had side effects. It is now fixed.
CallWindow: Uniformised C code with the ext video window code.
GmVideoWidget: Fixed logo resizing when coming back from fullscreen.