Add an alternative Polymer-based UI + persist window geometry between launches

...Also with easy switching between the UIs via a link for the interim.

@gaurave: PTAL at background.js (since you're in Apps V2 ;) )
@devoncarew: PTAL at everything else (since you're already in the loop on that); there will be conflicting changes between this and your UI tweaks: feel free to check yours first; I will resolve conflicts and upload an update here after that.