[WIP] Base init and attribute change events bypassing the event system if
there are no listeners.

Initial pass.

All tests pass:
✔ [Total]: Passed: 11009 Failed: 0 Total: 11100 (ignored 91)

TODO:

a) Add specific tests for init and attribute changes with/without listeners,
   and for init fireOnce behavior.

b) See if we can remove need for Model to override the private attribute change
   defFn, so we can remove one of the hacks which calls _defAttrChange with a false
   facade.