BHV-753: Waterfall onShowingChanged event in Panels.

Issue
-------
When a `Panels` control is hidden, followed by its parent being hidden, if this is reversed (parent is shown, followed by the `Panels` control being shown), the marquee is stuck.

Fix
----
This issue only occurs when hiding the parent after hiding the `Panels` control because hiding `Panels` does not set `display:none`; hiding the parent actually hides the `Panels` control, which causes the marquee transition to stop. When the parent is unhidden, the `onShowingChanged` is not yet waterfalled to the marquee because `Panels` has property `showing:false`, and when `Panels` is unhidden, the `onShowingChanged` is never waterfalled in the overridden `showingChanged` handler of `Panels`, so we make a call to the new `sendShowingChangedEvent` function.

Notes
-------
This PR is dependent on an Enyo PR: https://github.com/enyojs/enyo/pull/678

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>