FormView#getPropertyValue could support composite fields

Steps to reproduce

form:
  properties:
    name:
      $type: compositeField
      properties:         
        name:
          $type: textField
actions:
  commit:
    class: my.custom.action  

1. Use the form above.
2. Call FormView#getPropertyName from the action.

Expected results

I can retrieve the name field value as this is still a simple use case. 

Actual results

I can't access the field value, moreover FormView#getPropertyNames implemented by the same interface returns this field name.

Workaround

Avoid the complex field and add the field to the top of the form.

Development notes