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

@@ -65,15 +65,33 @@ 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
+     * return the form to be added to the panel
      */
-    this.create_show_form_callback = function (panel_id, title, form_config) {
+    this.create_show_form_callback = function (
+        panel_id, title, form_config, form_creator=undefined
+    ) {
         return (e) => {
             logger.trace("enter show_form_panel", e);
 
             const panel = $(form_panel.get_form_panel(panel_id, title));
             if (panel.find("form").length === 0) {
-                const form = form_elements.make_form(form_config);
+                if (form_config != undefined && form_creator!=undefined){
+                    throw new Error("create_show_form_callback takes either a FormConfig or a function that creates the form");
+                }
+
+                var form;
+                if (form_config != undefined ){
+                    form = form_elements.make_form(form_config);
+                } else if (form_creator != undefined ){
+                    form = form_creator();
+                } else {
+                    throw new Error("create_show_form_callback takes a FormConfig or a function that creates the form");
+                }
                 panel.append(form);
                 $(form).find(".selectpicker").selectpicker();