Handle variant list in formatting functions

Summary:
Improve the formatting function of
KFileMetaData such that it will also handle
QVariantLists for each property, to make it more
robust

Test Plan: tests pass

Reviewers: ngraham, bruns

Reviewed By: bruns

Subscribers: apol, kde-frameworks-devel, Baloo

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D19445