Bhv 5137 Add renderOnShow option

Requested from HQ developers.

Issue
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Due to performance matter, app developers would like to adjust rendering timing between render on creating and render on actual showing.

Cause
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enyo.Control does not have an option to delay rendering


Fix
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Add renderOnShow option. If developer sets it true, 
automatically this control has showing:false and canGenerate: false
When we call setShowing(true) first time, it will start render.
Yes, you are right. `this.canGenerate` should be back to true. I missed it.
Thank you @aarontam