color-device: Create devices without a ready color manager

Every monitor should eventually have a corresponding color device. To
make sure this can work, we must handle situations where the color
manager didn't connect to colord yet, and thus isn't ready.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3904>

src/backends/meta-color-manager.c

@@ -60,6 +60,7 @@
 enum
 {
   DEVICE_UPDATED,
+  READY,
 
   N_SIGNALS
 };
@@ -288,6 +289,7 @@ cd_client_connect_cb (GObject      *source_object,
                     color_manager);
 
   priv->is_ready = TRUE;
+  g_signal_emit (color_manager, signals[READY], 0);
 }
 
 static void
@@ -506,6 +508,13 @@ meta_color_manager_class_init (MetaColorManagerClass *klass)
                   NULL, NULL, NULL,
                   G_TYPE_NONE, 1,
                   META_TYPE_COLOR_DEVICE);
+
+  signals[READY] =
+    g_signal_new ("ready",
+                  G_TYPE_FROM_CLASS (klass),
+                  G_SIGNAL_RUN_LAST, 0,
+                  NULL, NULL, NULL,
+                  G_TYPE_NONE, 0);
 }
 
 static void