kcmkwin/kwinscreenedges: only show KCM on Wayland

The features that this KCM lets you trigger barely work on X11 and
really only work properly on Wayland.

Unfortunately we don't have the ability to have separate settings
between X11 and Wayland, and for this reason we removed a default left
touch edge in 3df79855 a year and a half
ago because it was too buggy on X11.

This commit takes the next step and hides the KCM entirely on X11, so
you can only use it on Wayland where it actually works properly.

One potential downside is be that any users who are actually using it
on X11 will lose the ability to use a GUI to configure these features
unless they go to the Wayland session to do it. However the number of
such users is likely small given how the feature does not really work on
X11.

However the upside is that Wayland users who can see the KCM can
actually use the features that it lets you configure because they work
properly there.