Removed the `form.widget()` directive and instead used an adapter.

That was done by restricting the adapter
to the `query` field on `ICollection` (using `z3c.form.util.getSpecification`).
This was made so that `plone.app.widgets`
can actually override the widget to use the `p.a.widget` compatible variant
(for now included in `p.a.widgets`).