This patch is the first of a series to rationalize the names of the text-input
related widgets.

See #7031

Callers can manually validate by calling validate(), or tell the Form to
validate on every change by setting the `autovalidate` parameter.

Fixes https://github.com/flutter/flutter/issues/7219

Fixes https://github.com/flutter/flutter/issues/7017

FormField is now a widget that can contain any type of field. Input no
longer has special code to handle form fields. Instead, there is a
helper widget InputFormField for using an Input inside a FormField.

Fixes https://github.com/flutter/flutter/issues/6097 and based on
feedback from the same.

Also, add more dartdoc to widgets.dart.