xwidget: Pass JavaScript return value to optional callback procedure

xwidget: Pass JavaScript return value to optional callback procedure

* lisp/xwidget.el (xwidget-webkit-execute-script): Accept optional
callback argument.
(xwidget-webkit-callback): Handle "javascript-callback" event type.
* src/xwidget.c (xwidget-webkit-execute-script): Accept optional
argument FUN, a Lisp procedure to execute on the JavaScript return
value.
(store_xwidget_js_callback_event, webkit_javascript_finished_cb,
webkit_js_to_lisp): New procedures.