Optional.
Extract SmileyDownloader into its own file.
Make a "Smiley Extractor" app to package up smileys to ship in the framework bundle.

In the meantime:

* Added NSProgress support to downloadMissingImageData.
* Indexed Smiley.text attribute.
* Gave Smiley and SmileyMetadata classes the @objc treatment so they're usable outside their framework.