[Extractor] Move property conversion from external process to calling job

Summary:
Using integer keys is more compact, and allows to remove the
KFileMetaData::PropertyInfo code from the temporary extractor.

Doing the conversion from integer to QString keys after serialization also
allows shared string data, as it uses the instances from PropertyInfo::name()
instead of inherently non-shared QStrings from deserialization.

Depends on D20956
Depends on D20738

Test Plan: ctest

Reviewers: Baloo, astippich, ngraham

Reviewed By: astippich

Tags: Baloo

Differential Revision: https://phabricator.kde.org/D20960