Use platform detection from KWorkSpace in KCMShell
Summary: In a Wayland session, KCMs need to use the wayland platform plugin.
Test Plan: kcmshell5 kscreen now works properly.
Reviewers: Plasma, romangg
Reviewed By: Plasma, romangg
Subscribers: romangg, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D11739