The `touched` property was never meant to support two-way binding; a
control should not be able to dictate that a field is no longer touched.

* The `touched` input represents the touched state of the field.
* The `touch` output allows a control implementation to indicate when
  the bound field is touched.

Note the distinction is that the `touch` output indicates _when_ the
field is touched, and not _whether_ the field is touched.