Bhv 5129 derekanderson

Added two new events BeforeShow and BeforeHide to ensure that implementations that already expect Show and Hide to be called after the popup state is altered.

Moved original showingChanged code into showHideMethod. Modified showingChanged to call showHideEvent (which was moved out of the original showingChanged method) before and after calling showHideMethod.

This PR doesn't alter the current ability for the chain to be called, but needs to go in before moonstone or onyx, to ensure that showHideMethod is called when needed.

Enyo-DCO-1.1-Signed-off-by: Derek Anderson <derek.anderson@lge.com>