[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