my-perf-box: split out the collection code so it doesn't depend on Polymer

This means we can start collecting stats immediately, even though it
takes time to load all of Polymer.
Separate major apps' initial HTML off from their vulcanized stuff
Add a placeholder loading state for the major apps
Improve pre-loading, avoid duplicating performance collector script
Use somewhat more realistic placeholder images and placeholder HTML

Yes, the diagonal lines look awful; they're there to make the
transition from image to HTML clearer during development.
Allow real requestFileSystem, not just webkitRequestFileSystem