- Default buttons in dialogs are now appropriately styled.
- When launching deja-dup with an existing hidden operation dialog
  (like happens when an autolaunched backup starts), we'll show the
  dialog instead of the preference window.
- When lauching an operation dialog while the preferences window
  is open, we mark the dialog as a modal child window, so the window
  manager can pair them correctly.
- When deja-dup is terminated unexpectedly, try harder to kill child
  duplicity processes.