Implemented as a separate mod to avoid further cluttering the monolithic core mod.
Play time, session count, and endgame freeze are always tracked.
Time is tracked during pauses as well (RTA rules).
A setting (default off) enables displaying the timer.
The timer is displayed in a text HUD in the upper right corner of the screen in sexagesimal format.
If play session count is not exactly 1, it's displayed in brackets (speedrun category rules may require exactly 1 session).
Timing starts when the server first starts ticking, and ends when the player right-clicks the final ghost (last meaningful input).
The timer HUD is white, which fits the game's visual theme reasonbly well, while standing out against the black sky much of the time, especially after right-clicking the last ghost. It blends in with some white brickwork and the credits screen but this is probably not enough of a problem.
Resolves #53