Add PosterHead component

(Shadow needs to be fixed).
Add default values to Shadow( ) attribute.
Fix output of shadow for text-shadow CSS attribute.
Adding doc comments to Menu.
Debug some article headers.
PosterHead now also checks on $subtitle in an article header.
Add PosterHead in the DbDWebsite example, replacing the header poster
image inside the article. Shadows need to be fixed and poster image
should not contain text. Use another image for that with another url in
the article head?