cursor: Disable if running under a wayland session

This plugin is harmful in a wayland session because it tells XWayland
to never show a cursor which results in mutter always using the
default cursor on X clients, e.g. the cursor doesn't change to a caret
on text widgets and doesn't change to the resize cursor on window
edges for those clients' windows.

The reason why the plugin doesn't work is that it uses the GDK device
manager to enumerate devices which only knows about the virtual
XWayland devices and then tries to use those devices to ask for idle
monitors on them through the gnome-desktop API which calls out to
mutter through DBus, but mutter doesn't know about those devices since
it's managing the devices itself.

Basically, this plugin's functionality needs to be implemented by
mutter itself when running as a native wayland compositor.

https://bugzilla.gnome.org/show_bug.cgi?id=738009