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".

#!/usr/bin/env python
# -*- coding: utf-8 -*-

# This file is part of Beremiz
# Copyright (C) 2019: Edouard TISSERANT
#
# See COPYING file for copyrights details.

from __future__ import absolute_import
from svghmi.svghmi import *