SVGHMI: add static file serving feature.

Added two buttons to add and remove files.
Files are stored in project inside a directory named "static",
itself located in svghmi CTN instance files. Exemple :
exemples/svghmi_csv_json_img_table/svghmi_0@svghmi/static/
Files are transfered as "extra_files", but with a prefix
added to their name to avoid conflics.

#!/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 svghmi.svghmi import *