Fix polymer UI menu - part #1

This CL fixes the Polymer UI not showing (crashing). It also (or mainly) fixes spark-selector, spark-menu, spark-menu-button, and the menu instantiation and hooking up in the Polymer UI, such that the event with the correct information about what menu item was clicked now bubbles up to the right receiver (SparkPolymer) in a better way than before. A that point there happen to be some problems: I'm getting weird exceptions in the most obscure places. I'm not sure, however, whether this is purely a problem in the new code, or it is somehow related to recent updates in the used packages.

I'd like an early review for this, because this unblocks further development related to the Polymer UI. I'll chase down those exceptions ASAP.

@terrylucas