Some cleanup, and get the thumbnails a little more working.
Ignore the built directory, which I think is a hold-over from a previous version of the code…
Make the tiles have a type of "thumbnail", and switch on that instead of the id "tiles".
(Thanks for the idea Jack! :)
Only store a very small set of things in the storage object.

Specifically the id and size.  We should do this because the rest of the
information should come from the available apps in the add-on.  Yes, this
means that we're not caching the contents, but I _think_ that's okay for now.
Make things jshint-happy, and assorted cleanup.
Get the tiles looking a little more like the previous tiles.