Add command to browse xwidget history

Add command to browse xwidget history

* doc/emacs/misc.texi (Embedded WebKit Widgets)
* etc/NEWS: Document `xwidget-webkit-browse-history'.

* lisp/xwidget.el (xwidget-webkit-mode-map): Bind "H" to
xwidget-webkit-browse-history.
(xwidget-webkit-import-widget): Set last session buffer correctly.
(xwidget-webkit-browse-history): New command.
(xwidget-webkit-history--session): New variable.

(xwidget-webkit-history--insert-item)
(xwidget-webkit-history-select-item)
(xwidget-webkit-history-reload): New functions.

(xwidget-webkit-history-mode): New major mode.