clutter/backends: Rename master and slave devices

Just because X11/XI uses a particular terminology doesn't mean we
have to use the same terms in our own API. The replacement terms
are in line with gtk@1c856a208, which seems a better precedent
for consistency.

Follow-up to commit 17417a82a5.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1425

src/backends/native/meta-virtual-input-device-native.c

@@ -696,7 +696,7 @@ meta_virtual_input_device_native_constructed (GObject *object)
   virtual_evdev->device =
     meta_input_device_native_new_virtual (virtual_evdev->seat,
                                           device_type,
-                                          CLUTTER_INPUT_MODE_SLAVE);
+                                          CLUTTER_INPUT_MODE_PHYSICAL);
 
   stage = meta_seat_native_get_stage (virtual_evdev->seat);
   _clutter_input_device_set_stage (virtual_evdev->device, stage);