Compatibility with older python3-nautilus

[nteodosio]

Quoting from devhelp (sorry, couldn't find a web document to link to), migrating Nautilus section:

The get_file_items, get_file_items_full, get_background_items a get_background_items_full methods of Nautilus.MenuProvider no longer take the window argument. Remove it from your implementations.
If you need to keep supporting older versions of Nautilus, you can use variadic arguments:

  def get_file_items(self, *args):
      # `args` will be `[files: List[Nautilus.FileInfo]]` in Nautilus 4.0 API,
      # and `[window: Gtk.Widget, files: List[Nautilus.FileInfo]]` in Nautilus 3.0 API.
      files = args[-1]

[CLAassistant]

All committers have signed the CLA.

[fmoc]

Please document this workaround in the code base as well, so we won't forget in the future why the files are queried from the packed parameters.

[fmoc]

Your change looks good, but I'd like to have the reasoning documented in the code as well.

[fmoc]

Thanks!

[jbicha]

Here's the weblink for the devhelp documentation:

https://gnome.pages.gitlab.gnome.org/nautilus-python/nautilus-python-migrating-to-4.html