[MetadataProvider] Use fixed order for modified/accessed/created
Summary:
The widget concatenates the group and the translated label to determine
the sort order. Using the same group for modified/accessed/created makes
the order dependent on the translated label.
Reviewers: Baloo, Dolphin, meven, ngraham, astippich
Reviewed By: meven, astippich
Tags: Baloo
Differential Revision: https://phabricator.kde.org/D20864