Make Composite's onChange event part of its event queue

[fjvallarino]

Composite's onChange event is now sent to its handleEvent function, not to its parent; the type of the generated event was updated to reflect this change. The rationale is that since onInit is sent to handleEvent, having onChange sent to its parent was confusing. At the same time there was not an easy way in handleEvent to know when the model changed.

Widgets that want to report model changes to its parent can use Report/RequestParent; an example can be found in ColorPicker.