ENH: allow to provide a function that creates the form to create_show_form_callback

Summary

Allow to supply a function that creates the form instead of a config.

Test Environment

unit test

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src/core/js/form_panel.js

@@ -65,7 +65,8 @@ var form_panel = new function () {
     };
 
     /**
-     * Creates a callback function that toggles the form panel which
+     * Creates a callback function that creates the form inside 
+     * the form panel when called for the first time.
      *
      * You may supply 'undefined' as form_config and use form_creator
      * instead which is supposed to be a function without argument that