Make "open in new window" from an xwidget's context menu work

Make "open in new window" from an xwidget's context menu work

* doc/lispref/commands.texi (Xwidget Events): Document new event type.
* doc/lisprefdisplay.texi (Xwidgets): Document new argument to
make-xwidget, and new function.
* etc/NEWS: Document changes.
* lisp/xwidget.el: Bind xwidget-display-event to
xwidget-webkit-display-event.

(xwidget-webkit-import-widget): New function.
(xwidget-webkit-display-event): New command.
* src/keyboard.c (kbd_buffer_get_event): New event type.
(make_lispy_event): Handle XWIDGET_DISPLAY_EVENTs.
(syms_f_keyboard): Define new symbol.
* src/termhooks.h (enum event_kind): New enum XWIDGET_DISPLAY_EVENT.

* src/xwidget.c (webkit_create_cb)
(store_xwidget_display_event)
(webkit_ready_to_show)
(webkit_create_cb_1, webkit_create_cb)
(Fset_xwidget_buffer): New functions.

(Fmake_xwidget): Add internal argument RELATED and connect create
signal.
(syms_of_xwidget): Define now subrs.