Hide some of kwin kcm on mobile

Summary:
- Window decorations, useless, since we use maximized and borderless
  windows
- Virtual Desktop, odd concept to be useful on mobile
- KWin Effects, doesn't have much of usecase on mobile

Reviewers: nicolasfella, davidedmundson

Reviewed By: davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D22336