macosx: remove unneeded performSelectorOnMainThread

setActiveVideoPlayback is always called on the main thread nowadays..
macosx: remove unneeded libvlc_Quit and add some comments for termination
macosx: set a proper main window frame on termination

The frame is used which was set before video playback has been started.
These are the values the user will expect when restarting VLC.

close #11373