Starrating now inherits from delite/FormValueWidget, and uses separate DOMNodes for left and right half of stars

Implements enhancements requested in https://github.com/ibm-js/deliteful/issues/145 and https://github.com/ibm-js/deliteful/issues/1
Thanks, this is fixed with commit https://github.com/sbrunot/deliteful/commit/5f61081491d09759ca2cf43e38d24447999278f9.