addPopupFilter(fnFilter) {
			if (typeof fnFilter === "function") {
				_aPopupFilters = _aPopupFilters.concat([fnFilter]);
			}
		}
Filter functions can be added to validate open popups.
sap.ui.dt.util.getNextZIndex() will return a z-index value greater than validated and less than invalidated popups.
@param {function} fnFilter Filter function to be added
@public