Support for inheritance of relativecontentpath in sitemapitem and referencesitemapitem for sitemenu to ease changing structures

For sitemap items, we would like to make possible a notation like:

sitemap
`news (hst:relativecontentpath=news)
` ** (hst:relativecontentpath=${parent}/${1})

for sitemenu items we'd like to make it possible to:

sitemenu
`News (hst:referencesitemapitem=news)

The ${parent} part gets filled in by the value of the same property of the parent