Adding getStaticOverlay method.

This will be used by overlays, e.g. to display layer attribution.

src/ol/Map.js

@@ -503,6 +503,13 @@ ol.Map.prototype.getMapOverlay = function() {
     return this.mapOverlay_;
 };
 
+/**
+ * @returns {Element} the static overlay element
+ */
+ol.Map.prototype.getStaticOverlay = function() {
+    return this.staticOverlay_;
+};
+
 /**
  * @export
  */