This adds a framework for running GUI autopkgtests using dogtail, Xvfb
and Python's unittest module. It also adds tests for some criteria
given in upstream's TESTING file:

  - whether the GUI starts up with no messages on stderr
  - whether the GUI actually cleans files (reusing the test file set
    from the commandline version tests)
  - whether unsupported files are handled as intended in cleaned archives
  - whether the status bar displays the messages set in the GUI code

Translations are less straightforward to test for now.