Add support for hiding the Play/Pause toolbar in W2gSync

See: How to use W2gSync - #29 by user_da7800b1aa763b7

The green toolbar is a little bit irritating while watching a video. So it would be very nice, if toolbar could be automatically hidden, after the the mouse disappeared due to inactivity (like in YouTube). As soon as the mouse is moved again, the toolbar becomes visible again.