SVGHMI: add a way for widgets and other yslt2 files to add css definitions to resultng page.
#!/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 *