[Plasma Quick] Add WaylandIntegration class
Summary: It allows us to get rid of DialogShadows::waylandPlasmaShellInterface().
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: kde-frameworks-devel
Tags: Frameworks
Maniphest Tasks: T12496
Differential Revision: https://phabricator.kde.org/D26491