Hide the marshallers

The marshallers we use for the signals are declared in a private header,
and it stands to reason that they should also be hidden in the shared
object by using the common '_' prefix. We are also using some direct
g_cclosure_marshal_* symbol from GLib, instead of consistently use the
clutter_marshal_* symbol.

clutter/clutter-behaviour-path.c

@@ -233,7 +233,7 @@ clutter_behaviour_path_class_init (ClutterBehaviourPathClass *klass)
                   G_SIGNAL_RUN_LAST,
                   G_STRUCT_OFFSET (ClutterBehaviourPathClass, knot_reached),
                   NULL, NULL,
-                  clutter_marshal_VOID__UINT,
+                  _clutter_marshal_VOID__UINT,
                   G_TYPE_NONE, 1,
                   G_TYPE_UINT);