BHV-686: moon.ExpandablePicker - Add multi-select option

We can't perform an array.join directly as the stringified value of the selection is not what we want, but could we consider doing something like this? Not sure if this helps to decrease any of the complexity:

`var selectionToString = function(inValue, inIndex, inArray) { return inValue.getContent(); };
		var selectedStrings = this.selected.map(selectionToString);
		str = selectedStrings.sort().join(", ");`