Introduce Time Sources

Looks like we want to have Time Sources as trait objects for the Timer, for both real time and game time. The game time one should probably be optional and the real time one could be the atomic clock, manually set for unit tests, or some kind of synchronization.

I'd really like to have a way to manually get time stamps from the time source for the hotkey system, and once the hotkey goes through, this time stamp is then used for processing the times. I'm not entirely sure how that fits in, as it should be able to be used with any time source (or rather with any pair).

No files or tests are mentioned. Start by locating the Timer and hotkey system, then trace how timestamps and real or game time are currently obtained; done means defining the required time-source behavior, including optional game time and manually controlled test time, without leaving the hotkey integration unresolved.

Written by the indexing model from the issue text.