fdo#76754 Add Positions by StyleSheetFamily to IndexedStyleSheets

Change-Id: I4eade2d00d145d8f65ccd70a1c6bbd0a134a1ad5
Reviewed-on: https://gerrit.libreoffice.org/10346
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>