Don't let button controls interfer with handlers.

This change involves removal of the map's eventsDiv and introduces an OpenLayers.Events.buttonclick component that adds a buttonclick event which makes sure that only events that are not related to clicking a button propagate. This allows button controls to be on the map's viewPortDiv again.

lib/OpenLayers/Layer/EventPane.js

@@ -108,7 +108,7 @@ OpenLayers.Layer.EventPane = OpenLayers.Class(OpenLayers.Layer, {
         }
 
         if (this.isFixed) {
-            this.map.eventsDiv.appendChild(this.pane);
+            this.map.viewPortDiv.appendChild(this.pane);
         } else {
             this.map.layerContainerDiv.appendChild(this.pane);
         }