honor the extractMetaData flag

Summary:
Metadata was extracted unconditionally before.
Only do so if it is actually requested.

Reviewers: Baloo, bruns, ngraham

Reviewed By: Baloo, bruns, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D29199