Add basic action definition

This is an empty placeholder, all it does right now is triggering a
warning each time DokuWiki tries to load it.

action.php

@@ -0,0 +1,23 @@
+<?php
+/**
+ * Tabpage plugin: Shows links to children pages as tabs on parent page
+ *
+ * @license    BSD 2-Clause
+ * @author     Antoine Le Gonidec <vv221.dokuwiki@dotslashplay.it>
+ */
+
+// must be run within Dokuwiki
+if ( ! defined('DOKU_INC') ) {
+	die();
+}
+
+class action_plugin_tabpage extends DokuWiki_Action_Plugin {
+	/**
+	 * Registers a callback function for a given event
+	 *
+	 * @param Doku_Event_Handler $controller
+	 */
+	public function register(Doku_Event_Handler $controller) {
+		trigger_error('register() not implemented in '.get_class($this), E_USER_WARNING);
+	}
+}