Add game time display

[jaapdeheer]

If we could display the game time somewhere (how long has the game been going), in replay playback and ideally during normal play, we could communicate things like "this replay mismatches at second 12:34". That would help debugging.

[xezon]

This is not just for debugging, but for gameplay and observing in general. It is somewhat important to see game time, for example to gauge No Rush 10.