refactor and reuse existing dialog to add potential domain entry

Change-Id: Ib884931f8ccc62aad9b3e92ecf93d1da7ffe607b
Reviewed-on: https://gerrit.libreoffice.org/69765
Tested-by: Jenkins
Reviewed-by: Xisco Faulí <xiscofauli@libreoffice.org>