fnOpenPopover = function() {
					var oPopover = that._getPopover();
					that._openPopover(oPopover, oThisButton);
				}
Gets the facet button for the given list (it is created if it doesn't exist yet).
The button text is set with the given list title.

@param {sap.m.FacetFilterList} oList The list displayed when the button is pressed
@returns {sap.m.Button} The button for the list
@private