st-widget: Move reactivity tracking to StWidget, use "insensitive"

This lets use remove another few pieces of code that do the tracking
manually.

https://bugzilla.gnome.org/show_bug.cgi?id=680426

src/st/st-button.c

@@ -446,15 +446,6 @@ st_button_class_init (StButtonClass *klass)
                   G_TYPE_INT);
 }
 
-static void
-notify_reactive_cb (StWidget *button)
-{
-  if (clutter_actor_get_reactive (CLUTTER_ACTOR (button)))
-    st_widget_remove_style_pseudo_class (button, "disabled");
-  else
-    st_widget_add_style_pseudo_class (button, "disabled");
-}
-
 static void
 st_button_init (StButton *button)
 {
@@ -464,9 +455,6 @@ st_button_init (StButton *button)
 
   clutter_actor_set_reactive (CLUTTER_ACTOR (button), TRUE);
   st_widget_set_track_hover (ST_WIDGET (button), TRUE);
-
-  g_signal_connect(button, "notify::reactive",
-                   G_CALLBACK (notify_reactive_cb), NULL);
 }
 
 /**