Remove non-generic stuff in the PicoXSiteFactory confoguration and FreemarkerSkin

XSite registers for the PicoXSiteFactory the non-generic TagRule AddFirstChildClassToHeader in the PicoXSiteFactory, that adds a class tag with the value FirstChild to the first header element.

The FreemarkerSkin implementation adds a property centerClass to its page context with value Content3Column if it is the index page and otherwise with the value Content2Column.

Both behaviors are specific to a special HTML skin used in the example and do not really belong into core. The example's skin has to be updated with Freemarker's own functionality for the 3-column-design on the index page.