Allow controlling where xwidget-webkit stores cookies

Allow controlling where xwidget-webkit stores cookies

* doc/lispref/display.texi (Xwidgets): Document new function.
* etc/NEWS: Announce `xwidget-webkit-cookie-file' and
`xwidget-webkit-set-cookie-storage-file'.

* lisp/xwidget.el (xwidget-webkit-cookie-file): New user
option.
(xwidget-webkit-new-session): Set cookie storage file.

* src/xwidget.c (Fmake_xwidget): Create new context for
each unrelated widget.
(Fxwidget_webkit_set_cookie_storage_file): New function.
(syms_of_xwidget): Define new subr.