Make xwidget-events special and document xwidget callbacks

Users have always been supposed to use callbacks for handling
xwidget events, but for some reason this has not been
documented until now.

* doc/lispref/commands.texi (Xwidget Events): Document xwidget
callbacks and the special status of `xwidget-event's.

* doc/lispref/display.texi (Xwidgets): Add xwidget property
list functions to the concept index.

* lisp/xwidget.el: Make xwidget events special.
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