Includes a bit more modularization of the uploading/deletion code.

Known bugs:
* Uploads work in Firefox but not in Konqueror, that is a regression
  compared to the D5 version. An ahah.js bug? (Drupal isn't even invoked.)
* The AHAH submit-replacement behaviour won't attach to the new form,
  it seems. I experienced the same thing with the CCK "add more" button,
  let's have a look what goes wrong there.