Appends `prop` with `style` to layer styling

def _compose_style(self):
        """Appends `prop` with `style` to layer styling"""
        valid_styles = (
            'color', 'width', 'filter', 'strokeWidth', 'strokeColor',
        )
        self.styling = '\n'.join(
            '{prop}: {style}'.format(prop=s, style=getattr(self, s))
            for s in valid_styles
            if getattr(self, s) is not None
        )