Make `xwidget-display-event' a special event as well

Make `xwidget-display-event' a special event as well

* doc/lispref/commands.texi (Xwidget Events): Document that
`xwidget-display-event' is a special event, and that it should
be handled through callbacks.

* etc/NEWS: Update NEWS entry.

* lisp/xwidget.el (xwidget-webkit-new-session)
(xwidget-webkit-import-widget): Attach display callback.
(xwidget-webkit-display-event): Call display callback instead.
(xwidget-webkit-display-callback): New function.

* src/keyboard.c (make_lispy_event): Store
source information for XWIDGET_DISPLAY_EVENT correctly.

* src/xwidget.c (store_xwidget_display_event): Store
source of the display request.
(webkit_ready_to_show): Store source if available.
(webkit_create_cb_1): Store source if available.
(kill_xwidget): Remove dead widget from internal_xwidget_list.