Introduce new lag constant for category view mode toggle.

(Already removed the old lang constant in addlang.txt which has
already been added.)

References #254

lang/addlang.txt

@@ -1 +1 @@
-@define('SERENDIPITY_PHPVERSION_FAIL', 'Serendipity requires a PHP version >= %2$s - you are running a lower version (%1$s) and need to upgrade your PHP version. Most providers offer you to switch to newer PHP versions through their admin panels or .htaccess directives.');
+@define('TOGGLE_VIEW', 'Switch category view mode');