Move current logical monitor retrieval to the backends

The method used for getting the current logical monitor (the monitor
where the pointer cursor is currently at) depends on the backend type,
so move that logic to the corresponding backends.

https://bugzilla.gnome.org/show_bug.cgi?id=777732

src/backends/meta-backend.c

@@ -572,6 +572,12 @@ meta_backend_warp_pointer (MetaBackend *backend,
   META_BACKEND_GET_CLASS (backend)->warp_pointer (backend, x, y);
 }
 
+MetaLogicalMonitor *
+meta_backend_get_current_logical_monitor (MetaBackend *backend)
+{
+  return META_BACKEND_GET_CLASS (backend)->get_current_logical_monitor (backend);
+}
+
 void
 meta_backend_set_keymap (MetaBackend *backend,
                          const char  *layouts,