SVGHMI: allow i18n of formated strings of HMI:Messages. This was by construction impossible since formating was given as an argument. Now added optional "format" labelled element in HMI:Display, so that it can be translated, when labelled "_format".
These classes are intended for use server-side.
e.g. in a django view.py :
from pyjs.jsonrpc.django import JSONService, jsonremote