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.
Modified keyboard shortcuts

Shortcuts for sliders have been changed to use Ctrl modifier to avoid
screen reader conflict.
Slider keyboard shortcut improvements

Changed keynames again due to conflict with browser shortcuts. Now
using function keys.
New keyboard shortcut, offscreen text adjustment

Using F9 instead of F8 for rate adjustment due to screen reader
conflict.

Changed wording of offscreen text to read "audio narration" instead of
"media overlay", to make it more user-friendly.

Hide offscreen text when MO buttons are not active.

If volume goes down to mute, and user hits unmute, volume restores to a
low but audible level.
Bugfix

MO offscreen text was shown when it should have been hidden.