How to Measure Children with Cerebral Palsy's Perception of Competence and Social Acceptance? Preliminary Study to Determinate the Psychometric Properties of the Portuguese Version of the Dutch Pictorial Scale of Perceived Competence and Social Acceptance in Children with Cerebral Palsy [Como Avaliar a Percepção de Competência e Aceitação Social de Crianças Com Paralisia Cerebral? Estudo Inicial Para a Determinação das Propriedades Psicométricas da Versão Portuguesa da Dutch Pictorial Scale of Perceived Competence and Social Acceptance in Children with Cerebral Palsy]