(visible sheet) of an embedded spreadsheet,
instead of exporting always the first sheet.

Change-Id: I5c6f982b1b814a5f3830f08d247f4db3fdc3c384
Reviewed-on: https://gerrit.libreoffice.org/67733
Tested-by: Jenkins
Reviewed-by: László Németh <nemeth@numbertext.org>