This has the following consequences:
- fixes the build after commit 58b704ad
- take a ref to the NautilusFiles when the list is passed-in
- simplifies the code calling the dialog from NautilusPropertiesWindow
- simplifies the code in NautilusMimeApplicationChooser, since we don't
  have to special case the files != NULL case and build a GFile manually
  from the URI