Major changes in 42.beta:
- Port video widget from obsolete Clutter to OpenGL
- Move subtitles and audio track selection to its own menu
- Add shortcuts for mute, cycling through subtitles, toggling
  subtitles and seeking to specific places in the video
- Implement a enabled-by-default native MPRIS plugin to replace
  the Python MPRIS plugin
- Remove media-player-keys plugin that supported the old GNOME
  Settings Daemon API, now replaced by MPRIS
- Show more precise elapsed time when stepping frame-by-frame
- Implement "search by title" in OpenSubtitles plugin