* src/xwidget.c: Add a `query-on-exit' flag to determine whether we can

* src/xwidget.c: Add a `query-on-exit' flag to determine whether we can
kill xwidget when the buffer is killed without confirmation.
(Fset_xwidget_query_on_exit_flag): New function to set the `query-on-exit'
flag.
(Fxwidget_query_on_exit_flag): New function to retrieve the value of the
`query-on-exit' flag.
(Fmake_xwidget): Set the `query-on-exit' flag to t by default.
* src/xwidget.h (xwidget): Add new field `kill_without_query'.
* lisp/xwidget.el (xwidget-kill-buffer-query-function): Ask for
confirmation before killing the buffer if a xwidget is present only if its
`query-on-exit' flag is enabled.