Use a similar idea as for the Application events. Use the SwDocShell
to keep the XSinkCaller. Call the Close event from
SwXTextDocument::close().

Change-Id: Ie873238c5a966fc859d45b59f424ae0e9f4fbfc7
Reviewed-on: https://gerrit.libreoffice.org/55110Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>