Add button to play in the background

[dmint789]

I've tried playing videos in the background many times, and it's really not clear how to do this currently. Through trial and error I realized that I have to not be in fullscreen mode and then leave the app, at which point it will start playing in the background. It would be better to have a button for this.

Also, it would be nice to have a video pop-up option too, same as on NewPipe.