Allow playing videos as audio-only

[vixalien]

You can already do this on the other YTM apps. This would be useful to play audio only.

Also maybe detect power saver or data saver and choose to play audio only.

[AdrianVovk]

Some other situations where it would make sense to stop playing the video:

• If the app is off screen, on another workspace, etc
• Outside of the now playing UI. See the screenshot; IMO showing the video so small isn't useful

[vixalien]

heh nice, a polyphia fan.

That actually makes sense, but there's currently a technically issue that prevents this.

The most appropriate way to do this imo would be to select the streams we want in GStreamer, but there's an issue in GJS which prevents us from doing that.