[ModifiedFileIndexer] Use correct mimetype for folders, delay until needed

Summary:
mimeTypeForFile is quite expensive (much more than QFileInfo and fetching
the timestamps from the DB), and yields the wrong type for folders.

Reviewers: Baloo, ngraham, astippich, poboiko

Reviewed By: astippich

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D21706