BHV-753: Waterfall onShowingChanged event in Panels.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>
BHV-753: Update name of the function that sends showingChanged event.
Merge pull request #1170 from enyojs/BHV-753-aarontam

BHV-753: Waterfall onShowingChanged event in Panels.

Reviewed-By: Ben Combee (ben.combee@lge.com)