Preloading xwidget.el on xwidget builds?

[Po Lu]

xwidget.el defines various parts of the internal xwidget machinery, such
as the `xwidget-event' handler, and pieces of Lisp code that keep
xwidget display working correctly.

Using xwidget-related C functions without xwidget.el loaded can lead to
crashes and display problems, so I propose to preload that file on
xwidget builds.

Thanks.