Fixing PanZoomBar and Panel issues after #164.

For PanZoomBar, this fixes the slider behavior. Now the buttonclick listener argument also includes a buttonXY property, and PanZoomPanel does not need an Events instance for the zoombarDiv any more.
For Panel, this fixes events for panels outside the map. Just setting the element on the Events instance was no longer enough after e70569b2bb. Events::attachToElement is now used, and it needed to be modified to also work if the Events instance had no element previously.
Finally, I renamed the button property of the buttonclick listener argument to buttonElement, to not confuse it with the browser event button property, and added some more tests and documentation.

lib/OpenLayers/Events/buttonclick.js

@@ -15,6 +15,12 @@
  *
  * This event type makes sure that button clicks do not interfere with other
  *     events that are registered on the same <element>.
+ *
+ * Event types provided by this extension:
+ * - *buttonclick* Triggered when a button is clicked. Listeners receive an
+ *     object with a *buttonElement* property referencing the dom element of
+ *     the clicked button, and an *buttonXY* property with the click position
+ *     relative to the button.
  */
 OpenLayers.Events.buttonclick = OpenLayers.Class({
     
@@ -57,6 +63,11 @@ OpenLayers.Events.buttonclick = OpenLayers.Class({
      */
     completeRegEx: /^mouseup|touchend$/,
     
+    /**
+     * Property: startEvt
+     * {Event} The event that started the click sequence
+     */
+    
     /**
      * Constructor: OpenLayers.Events.buttonclick
      * Construct a buttonclick event type. Applications are not supposed to
@@ -100,25 +111,30 @@ OpenLayers.Events.buttonclick = OpenLayers.Class({
                 element = element.parentNode;
             }
             if (OpenLayers.Element.hasClass(element, "olButton")) {
-                if (this._buttonStarted) {
+                if (this.startEvt) {
                     if (this.completeRegEx.test(evt.type)) {
+                        var pos = OpenLayers.Util.pagePosition(element);
                         this.target.triggerEvent("buttonclick", {
-                            button: element
+                            buttonElement: element,
+                            buttonXY: {
+                                x: this.startEvt.clientX - pos[0],
+                                y: this.startEvt.clientY - pos[1]
+                            }
                         });
                     }
                     if (this.cancelRegEx.test(evt.type)) {
-                        delete this._buttonStarted;
+                        delete this.startEvt;
                     }
                     OpenLayers.Event.stop(evt);
                     propagate = false;
                 }
                 if (this.startRegEx.test(evt.type)) {
-                    this._buttonStarted = true;
+                    this.startEvt = evt;
                     OpenLayers.Event.stop(evt);
                     propagate = false;
                 }
             } else {
-                delete this._buttonStarted;
+                delete this.startEvt;
             }
         }
         return propagate;