make an accessor for setting isBaseLayer, because we need to trigger the 'changelayer' (assuming our layer has already been added to a map)

git-svn-id: http://svn.openlayers.org/branches/openlayers/2.0@1324 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Layer.js

@@ -226,6 +226,16 @@ OpenLayers.Layer.prototype = {
   /*                                                      */
   /********************************************************/
     
+    /** 
+     * @param {Boolean} isBaseLayer 
+     */
+    setIsBaseLayer: function(isBaseLayer) {
+        this.isBaseLayer = isBaseLayer;
+        if (this.map != null) {
+            this.map.events.triggerEvent("changelayer");
+        }
+    },
+    
     /**
      * @type String
      */