Polymer switchover

@devoncarew @terrylucas @dinhviethoa @gaurave @keertip - giving everyone a chance to speak up

This CL is just a starter to be refined in a further collective effort. It simply replaces spark.* with spark_polymer.* in the entire build process, and as the default UI launched in background.js. It also moves spark_polymer_ui.* to the top level directory -- to simplify things and eliminate the need for several workarounds to Polymer and pub bugs. No further merging and cleanup of Polymer-vs-non-Polymer UIs has been done yet. The "Switch UI" context menu item is still preserved in the interim.

The deployed code has problems: the Polymer UI loads and mostly works, but has problems (with Chrome 33.0.1750.5 dev):
1) The menu is displayed and the toolbar assumes its height.
2) The splitter isn't displayed.
3) The non-system dialogs in the menu (Git Clone and About) are toggled, but not visible (the backdrop is visible, though).

At least some of these seem to be related to "@host" style rules or entire style sheets within polymer elements not applied to the elements as they should: for example, when the "@host" rules are manually transferred for the splitter in the DevTools, it shows up and works.