runDialog: Disable restart command on wayland

Clients can be expected to deal with the WM going away temporarily,
but not the display server - so when running as wayland compositor,
a restart is generally a fancy way of killing the user session, and
there's little we can do about it except for preventing the user to
shoot herself in the foot by throwing an error.

https://bugzilla.gnome.org/show_bug.cgi?id=741665