Misc/Cleanup

* moving commonly used strings to defines
 * moving commonly reused/similar page generation functions to the parent
 * generally using consistent return types, more type hints and less strings
 * prevent browser context menu when right clicking on UI elements with their own context menus
 * fixed menu path for icons

includes/ajaxHandler.class.php

@@ -10,7 +10,7 @@ class AjaxHandler
     protected $params      = [];
     protected $handler;
 
-    protected $contentType = 'application/x-javascript; charset=utf-8';
+    protected $contentType = MIME_TYPE_JSON;
 
     protected $_post       = [];
     protected $_get        = [];