org.gnome.PowerManager has two properties Icon and Tooltip.  It would be good if these emitted change notifications so that the client side proxy can cache them.

Seems like something similar to the following is needed:
  GVariantBuilder *builder;
  GError *error;

  error = NULL;
  builder = g_variant_builder_new (G_VARIANT_TYPE_ARRAY);
  g_variant_builder_add (builder,
                         "{sv}",
                         "Icon",
                         g_variant_new_string (icon_string));
  g_dbus_connection_emit_signal (connection,
                                 NULL,
                                 object_path,
                                 "org.freedesktop.DBus.Properties",
                                 "PropertiesChanged",
                                 g_variant_new ("(sa{sv}as)",
                                                interface_name,
                                                builder,
                                                NULL),
                                 &error);