skins2: remove unnecessary notifyLayout()
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.
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