In today's implementation, notifyLayout() for a control leads to a whole
 layout rebuilt. Therefore, this function must be called _only_ if needed.

- Never needed in onResize() since we are already in the process of rebuilding
the whole layout. Calling notifyLayout() means the job is done twice.
- Never needed in onPositionChange() for the same reason as onResize().
- Not needed for a checkbox since already called in setImage() two lines above.