Replays: add prestart and start events back into replays

This requires saving the replay_start gamestate before the prestart
event (as opposed to after the start event) and enabling execution of
the events in replays again.
Replays: disable the play/stop buttons after end of replay

The diverse play buttons and the stop button should be disabled when
the end of the replay is reached (which is also how it is in 1.10).
Other replay buttons remain active.