* js: add support for keydown events
This will modify the player behavior even if the player element is unfocused.
Based on the YouTube key bindings, allow to
- toggle playback with space and 'k' key
- increase and decrease player volume with up / down arrow key
- mute and unmute player with 'm' key
- jump forwards and backwards by 5 seconds with right / left arrow key
- jump forwards and backwards by 10 seconds with 'l' / 'j' key
- set video progress with number keys 0–9
- toggle captions with 'c' key
- toggle fullscreen mode with 'f' key
- play next video with 'N' key
- increase and decrease playback speed with '>' / '<' key
* js: remove unused dependency 'videojs.hotkeys.min.js'
Support for controlling the player volume by scrolling over it is
still retained by copying over the relevant code part from the
aforementioned library.
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