Dolphin Places: Only show file results in Nepomuk Queries

Dolphin is a file manager, and shouldn't be concerned with non-file
resources like notes, contacts and other stuff.

Use FileQuery instead of Query.

This also results in a faster queries. With File Queries Nepomuk can
perform some internal optimizations.

BUG: 306029
REVIEW: 106274

Cherry-picked from 6892d39d