[Extractor] Use QDataStream serialization in place of cooked one

Summary:
There is no need to reinvent serialization of Qt data types, just use
the one provided.

Depends on D16523

Reviewers: Baloo, Frameworks, ngraham, poboiko, lbeltrame

Reviewed By: lbeltrame

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D16524