Enabled a Timestep slider for replays and only there

We discussed https://github.com/cerebrum/dr/blob/master/games-strategy/openra/files/README.gentoo in IRC and @Cerebrum advises people to change Game.Timestep to fast-forward replays. This is a good use-case, but fatal for network play if people forget to reset it to 40 which will cause jittering. I added the GUI speed slider derived from experiments #2058 here and removed Timestep from `settings.yaml` to make sure it will never break.