[KFileMetaData] Add helper for XML encoded Dublin Core metadata

Summary:
Dublin Core metadata is used by several file formats, e.g. ODF,
MS Office 2007 (Office XML) and SVG. Add a helper which can be
used by the extractors.

Reviewers: Frameworks, astippich

Reviewed By: astippich

Subscribers: kde-frameworks-devel, Baloo

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D16488