Linktext investigation

As Michael requested, I did more in-depth investigation into the linktext
content model query.

It turns out there's a deeper issue apart from the differing domains. The
linktext element is one of two elements (along with shortdesc) that is
defined twice - once in the map module, once in the topic module. We've
discussed this before, and can't bring them in sync until 2.0, because they
use different class values (map/linktext vs topic/linktext) - if we change
that, we'll break either map or topic processors. This has been known since
before DITA 1.1.

Now, with 1.2, the content models have gotten out of sync. Proposal 12020
(easy reuse of small pieces of text) called for adding