Field : expose `forGetter` and `forVariable` factory ctors

This is designed to make it easier to use FieldEditors in a 'standalone'
fashion and is highly experimental.  It would be quite useful to be able
to use field editors in other parts of a ui outside of just the
ObjectEditor; in order to use a FieldEditor outside of an ObjectEditor
we need to make it easier / more robust for a user to create a Field.
This is all subject to change!