Enable linking to a HstSiteMapItem by refId

The advantage of this is that you can create a link to a 'fixed' refId instead of to some fixed 'path'.

Use case:

Suppose you have an English and Dutch website, both with their own sitemaps.

Dutch has item: 'kontakt' and English has 'contact'

Now, if you want to create a 'fixed' link in jsp, you want to use for example

<hst:link siteMapItemRefId='contact'>

Then, for the English site, you'll get 'general/contact' for example, and for Dutch you get '/algemeen/kontakt'

To have this feature, you need to configure on hst:refId on your HstSiteMapItem configuration and both 'kontakt' and 'contact' need the same refId.

Note that within a single SiteMap, no duplicate refId's are allowed. We'll throw an exception in this case during building the configuration model.