Add isRequired() to Form\Schema

This new method will make implementing a FormHelper context simpler.
Add fieldType to Form\Schema.

This new method will make implementing a form helper context easier.
Remove Schema::isRequired()

The validator provides this not the schema.
Remove remaining required references from Form\Schema.

The requiredness of a field comes from the field having a validation
rule.
Implement a FormHelper context class for Form\Form.

This allows FormHelper to work with Form instances.
Integrate FormContext into FormHelper.
Update based on review feedback and PHPCS errors.
Merge pull request #5453 from cakephp/3.0-form-context

3.0 form context