Making `xwidget-event' a special event

[Po Lu]

I would like to make `xwidget-event' and `xwidget-display-event' special
events.

This is because `xwidget-event' should never be grouped into key
sequences or echo, or the xwidget.el machinery could be confused.

For instance, if the user hits `C-x' and then immediately afterwards a
"load-finished" xwidget event is sent, xwidget-webkit will not receive
the "load-finished" xwidget event, which causes it to keep displaying a
loading indicator regardless of whether or not the load has actually
completed.

Users are supposed to use the `callback' property of the xwidget to
handle xwidget events anyway.  This hasn't been documented though, and
this deficiency in the documentation also exists in Emacs 28, so perhaps
the documentation there needs to be updated as well?

It also doesn't make sense to put an xwidget-event in a keyboard macro
or in unread-command-events.

Does this look OK?  If so, I'll make this change ASAP.

(Also, where would this change be best documented?)

Thanks.