[UnindexedFileIndexer] Pick up comment, tags and rating changes

Summary:
A folder should be reindexed (i.e. the terms should be updated) when
the file xattrs are updated, which also updates the ctime.

Depends on D21577

Test Plan:

1. stop the indexer
2. modify tags/rating/comment
3. restart the indexer

Reviewers: Baloo, ngraham, astippich, poboiko

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D21680