Mark playback streams in pavucontrol that have the DONT_MOVE flag and don't allow moving them
Currently, pavucontrol gives you the ability to choose a sink to move a stream to even when the streams are marked DONT_MOVE in which case nothing happens. As a user, I find this irritating and confusing and I think moving should just not be possible in that case because nothing happens anyway. The UI should disable the dropdown with the sinks to move to rather and show a helpful message for why moving isn't possible.