[Not really a pull request :)] File search box in the toolbar

My first attempt at fixing #655. This is not intended for merging yet. I just want to know if I am going in the right direction at all. Last time I dabbled in polymer it was still called web_ui. Please, pay attention to my TODOs as you might know the answers right away.

High level design:
- A common widget <spark-suggest-box> that takes user-supplied SuggestOracle as a source of suggestions
- A SearchOracle implementation for searching files
- The two come together as the new search box on the toolbar (spark_polymer_ui.*)