A base class for GTK audio and video widgets.
See its descendants: [class@ClapperGtk.Audio] and [class@ClapperGtk.Video].
Av defines a set of built-in actions:
```yaml - "av.toggle-play": toggle play/pause - "av.play": start/resume playback - "av.pause": pause playback - "av.stop": stop playback - "av.seek": seek to position (variant "d") - "av.seek-custom": seek to position using seek method (variant "(di)") - "av.toggle-mute": toggle mute state - "av.set-mute": set mute state (variant "b") - "av.volume-up": increase volume by 2% - "av.volume-down": decrease volume by 2% - "av.set-volume": set volume to specified value (variant "d") - "av.speed-up": increase speed (from 0.05x - 2x range to nearest quarter) - "av.speed-down": decrease speed (from 0.05x - 2x range to nearest quarter) - "av.set-speed": set speed to specified value (variant "d") - "av.previous-item": select previous item in queue - "av.next-item": select next item in queue - "av.select-item": select item at specified index in queue (variant "u") ```