video: Always pause if media-stream changed
When you have a GtkVideo
that's already playing some file, then
using gtk_video_set_file()
(or gtk_video_set_media_stream()
) will
not pause the old media stream if "autoplay" is not set.
What happens is that you then get a GtkVideo
which is playing both
sound from the old stream, as well as video and sound from the new one,
which is quite confusing.
when setting a new media-stream, we should just always put the previous one to paused to prevent any confusion.