Use case is that the presentation is started a few minutes in advance,
so the audience can check if they are at the correct room, seeing the
title side. Then previously the presentation had to be stopped and
started again, so the presenter had an idea how much time the talk took
so far. The side effect of this workaround was that the audience saw a
short flash while the presentation was not running.

Fix this by adding a button to just restart the timer, then it's not
needed to stop/start the presentation manually.

The two icons are just placeholders for now, real design is still
needed.

Change-Id: Id67e7e53a339ea5672dfb1c261f63d43466e9eff