Keep screen awake

[georgefst]

Firefox nightlies added support recently (EDIT: ah, but not on mobile) for the Screen Wake Lock API, which is already supported by the other major browsers.

This would obviously be useful, since some people (@ortk95) use very low screen timeouts. And there can easily be times when playing games where one doesn't need to touch the gamepad for, say, 30 seconds (Android's minimum timeout option).

I'll probably wait until support lands in a Firefox release before prioritising this.