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.
Hi @DavidUM, there is a typo with this method name. Are we sure we need this method (and the one above) actually, as we are setting the value of `this.canGenerate` in `renderOnShowChanged`.