Check for string lists and multi-values in property map

Summary:
Currently, KFileMetaData stores multi-values (e.g. multiple arists)
as multiple entries in the property map, while Baloo combines them to List
before serialization. More information in D19087 (your opinion is also appreciated there!)
or T8196.
Consider both cases such that the metadata retrieved by the baloo indexer and the file
indexer is the same.

Reviewers: mgallien

Reviewed By: mgallien

Differential Revision: https://phabricator.kde.org/D20502