A hst:sitemenuitem must be able to link to a sitemap item by refId and not only by path

Currently, a sitemenuitem can contain:

• hst:referencesitemapitem (string)

This is a reference by path.

However, for multi lingual sites, it makes sense to lookup a sitemap item by refId, as the sitemap items for different languages can be different urls (contact vs kontakt)

Thus, sitemenuitem will get an extra property:

• hst:refidsitemapitem (string)

The name is not the most elegant one, but is a little in line with - hst:referencesitemapitem which we need to maintain as it is used heavily.

The two properties must be exclusive: One of them should be defined, not both!