Add asynchronous Nepomuk resource retriever.

It seems that our poor performance under high I/O load is mainly caused by
using the synchronous Nepomuk API and Nepomuk using ionice due to FS
indexing. We use the synchronous API in quite a few places unfortunately,
but I've found only three on hot paths so far. This contains the fix for
the first one (action status updates in KMail), the other two are in the
message list (also observable by poor initial scrolling performance).