BHV-753: Create separate function for waterfallShowingChanged.

Issue
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For certain controls, such as `moon.Panels`, which override `showingChanged` to perform animations whenever `showing` changes, the `syncDisplayToShowing` should not be called but the waterfalling of `onShowingChanged` is still needed.

Fix
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The waterfalling of `onShowingChanged` has been separated out into a `sendShowingChangedEvent` function. The alternative would be to add another parameter to `showingChanged`, such as `preventSyncDisplay`, but would require some fiddling with the arguments object when calling `this.inherited` and having this separate function seemed like it could potentially be useful for other use cases. Please advise if there is a better way to handle this change.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>