feature: hidden fields in bueraucracy form from struct fields

This commit adds extra syntax which allow us to add struct fields to bureaucracy forms as hidden fields:

  struct_fieldhidden "products.product" "=value"

This only make sense with cosmocode#354

Author: Szymon Olewniczak

helper/fieldhidden.php

@@ -0,0 +1,22 @@
+<?php
+/**
+ * Allows adding a single struct field as a bureaucracy hidden field
+ *
+ * This class is used when a field of the type struct_fieldhidden is encountered in the
+ * bureaucracy syntax.
+ */
+class helper_plugin_struct_fieldhidden extends helper_plugin_struct_field {
+    /**
+     * Render the field as XHTML
+     *
+     * Outputs the represented field using the passed Doku_Form object.
+     *
+     * @param array     $params Additional HTML specific parameters
+     * @param Doku_Form $form   The target Doku_Form object
+     * @param int       $formid unique identifier of the form which contains this field
+     */
+    function renderfield($params, Doku_Form $form, $formid) {
+        $this->_handlePreload();
+        $form->addHidden($params['name'], $this->getParam('value') . '');
+    }
+}