[IMP] Add eventhandler for valuechange of counter

[FVMF]

The default counter object that comes with TTS is useful to use in mods. However, I use multiple counters and the value of some counters is dependent on the (combined) value of other counters. To keep them in sync at the moment, I have to use the global event handler onUpdate() or onFixedUpdate() to keep the counters in sync. These global event handlers are both CPU intensive. It would help if there would be a global event handler that is triggered whenever any counter changes value.