Add Q_PROPERTIES

Summary:
This isn't 100% complete yet, but enough to become useful for the two
Grantlee formatters consuming these objects. It should also be useful
for QML consumers (therefore using variant containers rather than strictly
typed ones).

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: mlaurent, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D20485