FXFormFieldTypeLabel no longer pushes subform when tapped.

This fixes an issue I had when trying to make a Field readonly.

This particular field's `valueClass` conformed to the `FXForm` protocol
which meant that a `UITableViewCellAccessoryDisclosureIndicator` was
shown and, when tapped, a subform representing this field would be
pushed onto the navigation stack.

This fix does two things:

* it abstracts the `isSubform` logic into a readonly property of
  `FXFormField`
* `isSubform` now always returns `NO` if the field's `type` is equal to
  `FXFormFieldTypeLabel`