PageRouter: find parents of items better

Summary: The PageRouterAttached is now able to locate a parent PageRouter by travelling up the visual parent hierarchy as well as the regular QObject parent hierarchy.

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: mart, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D29135