One selection layer per source layer

This way all styles can be defined on the source layer, by
defining a rule with a select renderIntent. It also will make
addition and removal of layers easier while the select control
is active.

src/ol/layer/vectorlayer.js

@@ -349,6 +349,14 @@ ol.layer.Vector.prototype.getVectorSource = function() {
 };
 
 
+/**
+ * @return {ol.style.Style} This layer's style.
+ */
+ol.layer.Vector.prototype.getStyle = function() {
+  return this.style_;
+};
+
+
 /**
  * Get all features whose bounding box intersects the provided extent. This
  * method is intended for being called by the renderer. When null is returned,