Add Layer.setZIndex so that layers can add behavior when their div's z-index changes. Also, change Map.setLayerZIndex() to use it.

git-svn-id: http://svn.openlayers.org/trunk/openlayers@1611 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Layer.js

@@ -516,6 +516,14 @@ OpenLayers.Layer.prototype = {
         }
     },
 
+    /**
+    * @param {int} zIdx
+    * @private
+    */    
+    setZIndex: function (zIdx) {
+        this.div.style.zIndex = zIdx;
+    },
+
     /** @final @type String */
     CLASS_NAME: "OpenLayers.Layer"
 };

lib/OpenLayers/Map.js

@@ -261,9 +261,9 @@ OpenLayers.Map.prototype = {
     * @private
     */    
     setLayerZIndex: function (layer, zIdx) {
-        layer.div.style.zIndex = 
+        layer.setZIndex(
             this.Z_INDEX_BASE[layer.isBaseLayer ? 'BaseLayer' : 'Overlay']
-            + zIdx * 5;
+            + zIdx * 5 );
     },
 
     /**