Media overlays improvements; added keyboard shortcuts.

Rewrote parts of the media overlays controller integration layer. Now
relies as little as possible on page refresh events, making playback
smoother and allowing section navigation to work without interruption.
Also fixes some page navigation bugs caused by multiple page refreshes
upon going to a new page.

Added access keys for mute/unmute and reset rate. Added keyboard
shortcuts for increase/decrease volume and increase/decrease rate.