Declare PropertyInfo as QMetaType

Summary:
Required for data driven unit tests, probably useful elsewhere.

No functional changes

Test Plan: ctests

Reviewers: Baloo, Frameworks, ngraham, astippich, apol

Reviewed By: apol

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D20031