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