selective suppression of fields in edit template

This changeset implements selective suppression of fields in the edit template field part. It consists of two changes:
* a new filter operator named "basename"; given a hierarchical tiddler name, such as $:/config/fields/my-foo-field it returns only the basename part of title, which is my-foo-field. This new filter is used in the following, second change.
* a small change to the filter expression in core/ui/EditTemplate/fields which removes those fields from the set which have a corresponding $:/config/fields/fieldname tiddler which is tagged as $:/tags/ExcludeEditField.