Cache thumbnail images

[apsun]

Right now caching is in-memory only, and in order to generate the thumbnails we resize the original images down. This feels wasteful (although I don't know how wasteful), so we should probably write the thumbnails we generate to Library/Caches to directly load them next time.

We would have to do this lazily on first use though, since from the app container we have no clue how big the keyboard will be.