This is simply an empty part, that is loaded and displayed when a
document contains an invalid part
- either because the handler for this part isn't available
- or because the embedded document is external (out of the store) and unavailable

The goal of the part is to show a placeholder in the document, to tell
about the unavailable part, but also to be able to save itself as it was,
so that it's possible to edit a document with a missing child document
without losing information.

Test documents:
test-kounavail.kwd points to an invalid mimetype (case 1)
test-kounavail2.kwd points to a non-existing external document (case 2)

David Faure <faure@kde.org>