Allow directory_listing template to be extended easier.

 * Add `object-tools` block similar to how change_form in django-admin works
 * Allow the directory_listing template from FolderAdmin to be overwritten

Overall this allows to add custom actions, e.g to link to a custom form
in the folder admin.
Add object-tools-items
Merge pull request #440 from EnTeQuAk/feature/allow_directory_listing_overwrite

Allow directory_listing template to be extended easier.