add an icon for calc's time number format

Change-Id: Ife32075210af738e0e677f62ccad1ce29b236700
Reviewed-on: https://gerrit.libreoffice.org/12962
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>