WICKET-5628 Introduce a marker interface for exception which are recommended to be handler by the framework

Rename the marker interface to start with 'I' as all other interfaces in Wicket
WICKET-5629 Add an HeaderItem for meta data tags such as <meta> or canonical <link>

Add checks for the passed arguments.
Escape the name and the value before rendering.
Remove unused constant - CANONICAL_LINK
WICKET-5630 Fix last button translation germany of wizard to 'Letzter'