FDO#83283 (part) help btn for Label Field select'n

Help button for Calc Label Field Selection

Change-Id: Ia3024992ff804be469cf32caf1a2b84675557415
Reviewed-on: https://gerrit.libreoffice.org/11208
Reviewed-by: Thomas Arnhold <thomas@arnhold.org>
Tested-by: Thomas Arnhold <thomas@arnhold.org>