Handle standard tel/sms/geo/mailto URIs in the contact viewer

This is a first step towards moving away from the custom URIs used for
those here so far. Doing this has multiple advantages:
- we don't need to figure out what to do in those cases, but can rely
on meanwhile existing standard infrastructure (which we largely do in the
default action handler already anyway, just with extra indirection)
- the generated HTML will also work in KMail (which uses this for embedded
vCards), but which doesn't handle the custom URIs (and thus seemingly
interactive elements aren't actually interactive).