Checkbox should not dispatch change event when selected property programmatically set [backwards compatibilty]

Reproduction:
1. Click on checkbox
2. Notice new message in TextArea saying that a change event was received
3. Click on Button "Programmatic Change"

Actual Results:

A change event is dispatched, which shows another line in the TextArea

Expected Results:

No change event should be dispatched. The TextArea should not change

Workaround: Use the click event instead

To be consistent with the rest of the components, Button should only dispatch a change event in response to user interaction. Programmatically setting the selected property should dispatch a valueCommit event instead. This fix should be relatively easy. Button.setSelected is called in response to user interaction and due to programmatic changes. Add a flag to setSelected so that it dispatches the correct event depending on the source of the change.