Added icons for conditional formatting

Change-Id: If2dd67ad08d93e020e0e68043f9270d387bd41d3
Reviewed-on: https://gerrit.libreoffice.org/7246
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>