Allow to set "breakpoints" on `Clock` & `Countdown`

[Byloth]

Allow to set callback functions to be called once reached specific timing while Clock or Countdown objects are ticking.

[Byloth]

Instead of adding to these classes methods and properties to handle this, consider defining a new class TimedBreakpoints (or something like this) to which pass a Clock or a Countdown to subscribe to.

This should allow both to keep these classes lightweight and the ability to replace the Clock or the Countdown in real time without the need of reinstanciate breakpoints from stratch.