Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'width') and self.width is not None:
            _dict['width'] = self.width
        if hasattr(self, 'height') and self.height is not None:
            _dict['height'] = self.height
        if hasattr(self, 'left') and self.left is not None:
            _dict['left'] = self.left
        if hasattr(self, 'top') and self.top is not None:
            _dict['top'] = self.top
        return _dict