Rename AddFirstChildClassToHeader to AddClassAttributeToFirstHeaderRule

The AddFirstChildClassToHeader rule is not at all generic and the class name does not follow the standard naming rules from SiteMesh. Therefore the current implementation is renamed to AddClassAttributeToFirstHeaderRule to reflect its real purpose and make the value of the added class attribute configurable instead of using a hard coded value. Keep the old class type as deprecated version around, now derived from the new implementation.