Make component id easily retrievable on components form

The new preprocess approach in Webform 4 is nice. The only trouble is that if I impliment a preprocess hook and loop through the rows, there's no way to retrieve the component id without resorting to some ugly string manipulation. This patch makes it possible to get the component id easily for each row, both from a php hook, and also from the client side.
See #1468944: Preprocess variables for components form