Change disclosure +/- icons to pointers

Change-Id: Ia42e2bc5120be294bc791dd2e2f06dd1fc2a2c8a
Reviewed-on: https://gerrit.libreoffice.org/12500
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>