Keyboard shortcut

[plakhin]

This PR adds keyboard key binding to start/cancel seek.
Currently the s key is bonded.

[plakhin]

@lhc70000, the initial idea was to bind the space key, the same YouTube does: on single press it cycles through pause/resume, on hold (repeated entry) it increases the speed (the onKeyDown provides isRepeat which would be useful in this case). However, I couldn't intercept the space key down event — it simply doesn't work, may be because the "pause" interceptor has the highest priority and stops event propagation, idk. Also, unfortunately, keys like "Shift" or "Meta" can't be used alone as they're treated as modifiers and must be used in combination with at least one any other key.