qobject_cast rather than dynamic_cast

Summary:
doesn't require RTTI and is faster

(I am not sure why there are casts at all instead of pointer members...)

Test Plan: builds

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22403