ENYO-2424: Modify onyx.Popup zIndex selection to pick adequately high z-index values for popups on top of other popups

Based on code suggested in bug by Chris Van Hooser

Enyo-DCO-1.1-Signed-Off-By: Ben Combee (ben.combee@lge.com)
Merge pull request #158 from enyojs/ENYO-2424-unwiredben

ENYO-2424: Modify onyx.Popup zIndex selection to pick adequately high z-...