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.

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# opcua/__init__.py

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from .client import OPCUAClient

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# class RootClass(object):

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#     XSD = """<?xml version="1.0" encoding="ISO-8859-1" ?>

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#     <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

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#     </xsd:schema>

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#     """

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#     CTNChildrenTypes = [("OPCUAClient", OPCUAClient, "OPCUA Client")]

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#