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

index.php

@@ -104,7 +104,7 @@ switch ($pageCall)
                     header('Location: '.$out, true, 302);
                 else
                 {
-                    header('Content-type: '.$ajax->getContentType());
+                    header($ajax->getContentType());
                     die($out);
                 }
             }