Implement Dropdown.bind

This closes select2#3239

Author: kevin-brown

src/js/select2/dropdown.js

@@ -25,6 +25,10 @@ define([
     return $dropdown;
   };
 
+  Dropdown.prototype.bind = function () {
+    // Should be implemented in subclasses
+  };
+
   Dropdown.prototype.position = function ($dropdown, $container) {
     // Should be implmented in subclasses
   };