Guarding the X11 calls not to appear in a wayland session

Summary:
All x11 methods got the x11_ prefix to be more obvious which parts
are x11-only. Added default implementation for wayland sessions, and
a note that activity switching using keyboard is a regression compared to x11

Reviewers: sebas, graesslin, bshah

Reviewed By: bshah

Subscribers: gladhorn, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D1963