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

js/ui/messageTray.js

@@ -799,10 +799,6 @@ const Notification = new Lang.Class({
             return;
 
         button.reactive = sensitive;
-        if (sensitive)
-            button.remove_style_pseudo_class('insensitive');
-        else
-            button.add_style_pseudo_class('insensitive');
     },
 
     setUrgency: function(urgency) {