Bhv 11232 Enforce condition statement for setShowing()

Issue
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If developer gives null or undefined as an argument for setShowing(), it makes a control to be invisible. It is natural behavior.
However if developer call setShowing(false) at this status, it shows a control.
Here is a logical bug which we did not expected.

Cause
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In Enyo 2.3.0, showingChanged() will waterfall 'onShowingChanged' event when 'was' - previous showing value- was 'true' or 'false'
However in Enyo 2.5.0 we use like follows

if (!was) this.applyStyle('display', this._display || ' ');

So 'false' after 'null' removed display: none and a control becomes visible.

Fix
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Enforce condition statement with double check - was and is -