color-manager: Add ready state

Ready means it has established the connection to colord and can operate.
Will be used by tests to make sure tests don't fail due to race
conditions when connecting to colord.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2164>

src/backends/meta-color-manager.c

@@ -74,6 +74,8 @@ typedef struct _MetaColorManagerPrivate
   GHashTable *devices;
 
   MetaDbusSettingsDaemonColor *gsd_color;
+
+  gboolean is_ready;
 } MetaColorManagerPrivate;
 
 G_DEFINE_TYPE_WITH_PRIVATE (MetaColorManager, meta_color_manager, G_TYPE_OBJECT)
@@ -203,6 +205,8 @@ cd_client_connect_cb (GObject      *source_object,
   g_signal_connect (monitor_manager, "monitors-changed-internal",
                     G_CALLBACK (on_monitors_changed),
                     color_manager);
+
+  priv->is_ready = TRUE;
 }
 
 static void
@@ -367,3 +371,12 @@ meta_color_manager_get_color_device (MetaColorManager *color_manager,
   monitor_id = generate_monitor_id (monitor);
   return g_hash_table_lookup (priv->devices, monitor_id);
 }
+
+gboolean
+meta_color_manager_is_ready (MetaColorManager *color_manager)
+{
+  MetaColorManagerPrivate *priv =
+    meta_color_manager_get_instance_private (color_manager);
+
+  return priv->is_ready;
+}