fdo#84938: replace TOOLBOXITEM_ constants with enum

Change-Id: I08c4a456f9e80f70719ca8c3ad5c0f0d2d8282f6
Reviewed-on: https://gerrit.libreoffice.org/12258
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>