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