reverting r12228 and r12185 (closes #3431)

git-svn-id: http://svn.openlayers.org/trunk/openlayers@12231 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Layer/Markers.js

@@ -60,6 +60,22 @@ OpenLayers.Layer.Markers = OpenLayers.Class(OpenLayers.Layer, {
         OpenLayers.Layer.prototype.destroy.apply(this, arguments);
     },
 
+    /**
+     * APIMethod: setOpacity
+     * Sets the opacity for all the markers.
+     * 
+     * Parameter:
+     * opacity - {Float}
+     */
+    setOpacity: function(opacity) {
+        if (opacity != this.opacity) {
+            this.opacity = opacity;
+            for (var i=0, len=this.markers.length; i<len; i++) {
+                this.markers[i].setOpacity(this.opacity);
+            }
+        }
+    },
+
     /** 
      * Method: moveTo
      *