backends: Pass seat ID to evdev ClutterDeviceManager

Clutter assumed seat0 which is most usually, but not always correct.
Add an evdev-backend specific function to allow passing the seat
that will be used for ClutterDeviceManager construction, which we
already obtain in MetaLauncher.

https://bugzilla.gnome.org/show_bug.cgi?id=778092

src/backends/native/meta-launcher.c

@@ -545,6 +545,8 @@ meta_launcher_new (GError **error)
   self->kms_fd = kms_fd;
   self->kms_file_path = kms_file_path;
 
+  clutter_evdev_set_seat_id (seat_id);
+
   clutter_evdev_set_device_callbacks (on_evdev_device_open,
                                       on_evdev_device_close,
                                       self);