kms: Add 'device-added' signal

This signals when there was a mode setting device added.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3746>

src/backends/native/meta-kms.c

@@ -34,6 +34,7 @@
 enum
 {
   RESOURCES_CHANGED,
+  DEVICE_ADDED,
 
   N_SIGNALS
 };
@@ -336,6 +337,8 @@ meta_kms_create_device (MetaKms            *kms,
 
   kms->devices = g_list_append (kms->devices, device);
 
+  g_signal_emit (kms, signals[DEVICE_ADDED], 0, device);
+
   return device;
 }
 
@@ -452,6 +455,15 @@ meta_kms_class_init (MetaKmsClass *klass)
                   G_TYPE_NONE, 1,
                   META_TYPE_KMS_RESOURCE_CHANGES);
 
+  signals[DEVICE_ADDED] =
+    g_signal_new ("device-added",
+                  G_TYPE_FROM_CLASS (klass),
+                  G_SIGNAL_RUN_LAST,
+                  0,
+                  NULL, NULL, NULL,
+                  G_TYPE_NONE, 1,
+                  META_TYPE_KMS_DEVICE);
+
   meta_thread_class_register_impl_type (thread_class, META_TYPE_KMS_IMPL);
 }