Create icon cache

[zenangst]

The application should be able to display the application icon along-side the application name.
I propose making a separate class for indexing, creating and caching application icons that can be used when rendering the views. Preferably it could use the application bundle as the cache key.
In the initial implementation, it would be good enough to only cache during the first operation and then reuse the results in an in-memory cache. In the long-run, it would be nice to be able to persist the cache between launches.