Make seek bar in demo player toggable/hide automatically

This closes #1267; the seekbar in demoplayer will hide after a few seconds, as described by the belonging issue.

It is tested and works fine, exactly as it is supposed to.
I'm not quite sure if it is the best way to delete it, but it causes a bug which makes the seek bar fill the whole menu if the menu AND the seek bar are active at the same time. I tested it, the bug was fixed and no other issues appeared.