Simplify behaviors by introducing behavior-state-changed event

[okke-formsma]

This is a prototype for a behavior-state-changed event, as suggested in #509

• other events can listen to behavior-state-changed events
• the behavior-binding functions now can capture the behavior-state-changed event, whereas the current lifetime of *bindings is not guaranteed

Please read this PR commit-by-commit. The last two commits are refactors of hold-taps and sticky-keys which show how this PR creates the ability to easily capture the behavior-state in behaviors.

I will "drip-feed" thse commits in in small pieces as well.

[innovaker]

See #523.

[okke-formsma]

We currently have the following events

• position_state_changed
  • keycode_state_changed
  • layer_state_changed

What is missing from this event flow are behavior bindings. This means they can not be observed like other events.

• position_state_changed
  • behavior_state_changed
    • keycode_state_changed
    • layer_state_changed

This PR adds the beahvior_state_changed event which is invoked just before an event is invoked.