environment: improve Object.toString for delegate objects

If a JS object has an 'actor' property pointing to a ClutterActor,
include the actor's toString() output in the delegate's toString()
output. Eg:

  js>>> Main.panel
  [object Object delegate for 0xff6080 StBoxLayout.menu-bar "panel" ("Activities")]

https://bugzilla.gnome.org/show_bug.cgi?id=646919

js/ui/environment.js

@@ -88,6 +88,15 @@ function init() {
         return St.describe_actor(this);
     };
 
+    let origToString = Object.prototype.toString;
+    Object.prototype.toString = function() {
+        let base = origToString.call(this);
+        if ('actor' in this && this.actor instanceof Clutter.Actor)
+            return base.replace(/\]$/, ' delegate for ' + this.actor.toString().substring(1));
+        else
+            return base;
+    };
+
     if (window.global === undefined) // test environment
         return;