Form_Basic: fix last_field, remove disable()

1. $form->last_field is no more used anywhere and incorrectly used with setting attributes trough addField() method.
2. $form->disable() tried to disable last added field, but ... it's not used anywhere. As result this is useless method. We can disable field by using $field->disable().
3. Maybe there should be method $form->disable(), but with completely different meaning - disable all form fields one by one. I'm not sure could it be useful or not.