* Put up the progress bar on startup, at least before retrieving the
  Packages file.

* Release files

* Better error-resistance. One failing package shouldn't shoot everything.
  (Partly done now that we have retriever error correction.)

* "Tasks". Only list udebs that have a certain control field. Use the contents
  of that control field as info in the list. Other udebs will be pulled in by
  dependencies.

* Investigte removing the version comparison code in is_installed(). Is
  there any reason for anna to upgrade installed udebs? Why was this code
  put in in the first place?