Revive 'Description' property for DublinCore metadata

Summary:
Property::Description did not make it into KF5, Property::Comment is used instead .
This patch revives 'Description' for the DublinCore metadata as it is more specific and avoids confusion.
Changes the property for xml, epub, odf and office documents

Test Plan: make test

Reviewers: Baloo, Frameworks, bruns

Reviewed By: Baloo, bruns

Subscribers: kde-frameworks-devel, bruns

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D12114