More dexterity-related fixes

This pull-request continues #26
If the portlet is configured to filter by portal_type, we also need to make sure that we don't depend on the existence of the archetype_tool.
Therefore, `_listTypesForInterface` in quick_upload now takes care of handling AT vs dexterity. Note: the signature has changed. Instead of taking an interface as argument (previously this was AT-specific), it takes a string, either "file" or "image". The method then looks up the correct interface for the requested type, based on whether AT or dexterity is present.

The `UploadFileTypeVocabulary` for the portlet has gotten simpler, since all distinction between AT and dexterity has been moved to `_listTypesForInterface`.
(I also did some pep8 / flake8 cleanup, so that the diff seems larger than it actually is)