Creates an object (menu, dialog, control, ...) from an XML node.
Should check for validity. {\it parent} is a higher-level object (usually window, dialog or panel)
-that is often neccessary to create the resource.
+that is often necessary to create the resource.
If {\bf instance} is non-NULL it should not create a new instance via 'new' but
should rather use this one, and call its Create method.