Additional Hotkey to play back video in the subtitle editor

[dennis531]

Currently, users can not play/pause the video in the subtitle input field. These changes solve this problem by introducing an alternate hotkey Strg + Alt + Space to play and pause videos.

Resolves #901

[lkiesow]

@Arnei, you approved. I see the effect that pressing Ctrl, then Alt, then Space doesn't work. Pressing Alt first, then Ctrl, then Space works just fine. I see this in both Firefox and Chromium. Not sure if this conflicts with a system-wide hotkey (nothing seems to happen). Does that work for you? If yes, feel free to merge this.

[Arnei]

@lkiesow for me both orders (Ctrl --> Alt, Alt --> Ctrl) work. I recall that you described this problem previously in #739 months ago, so I think this is likely a problem on your end (that more users could have anyway, so it may still be worth investigating).