Video: refactor volume control.


Improvements:
- `construtor-prototype` is used;
- `this` refers to `VolumeControl`;
- access to `state` via `this.state`;
- fix class names (`open` -> `is-opened`, `muted` -> `is-muted`, etc);
- remove dependencies between modules;
- add docstrings;
- keys UP/DOWN increase/decrease volume level on every element inside video container, but not for speed control and progress slider;
- fix `TABbing` logic;
- clean up `html`.

@jmclaus , @auraz  please review.