This is a pretty pointless shallow wrapper to GFileEnumerator. Just make
TrackerDataProvider offer GFileEnumerators. If anyone ever was in need for
implementing a TrackerEnumerator, they can do the very same by implementing
GFileEnumerator, and handing those from their TrackerDataProvider impl.

This also cuts a level of indirection in directory crawling (i.e. no
wrapping of GTasks in GTasks), and allows us to fully use GFileEnumerator
API (eg. g_file_enumerator_next_files will be handy).