qt: controller: Only react to external fullscreen changes
qt: controller: Only react to external fullscreen changes

And assume the playlist's "fullscreen" variable to be the reference
value.
Fix #17054

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc