Pressing "stop" should close the MPV window

[forteller]

When pressing "stop" in the Multiplex window you are taken back to the screen where you input a magnet link. So watching the file you where watching is over. But the MPV window stays, and if you start a new stream, it opens a new MPV window. When you press stop the currently used MPV window should be closed.

Thanks! :)

[pojntfx]

Hi! This is a good point - the larger rework ticket will probably include this, since MPV will no longer need to be managed externally: #12