Fix for: BHV-8484

Optimizations for Control initialization removed the unnecessary `showingChanged()` call that Panels had overloaded and expected. It also has an irregular handling of the `showing` flag as it can be `false` but still never apply the `display: none;` style property. This fix allows the expected behavior to continue.

Enyo-DCO-1.1-Signed-Off-By: Cole Davis (cole.davis@lge.com)