SVGHMI: add support for "enable expressions" with arbitrary variable name assignment.

HMI tree paths can be prefixed with a variable name "@name=/MY/HMI/VAR"
Widget declarations can end with a "#" followed by a JS expression that refers to name given to variables.
Widget is disabled if expression's result is false.

Commit includes some more-or-less related generated code refactoring, that should simplify extending widget's variables attributes.

# opcua/__init__.py

from __future__ import absolute_import

from .client import OPCUAClient

# class RootClass(object):
#     XSD = """<?xml version="1.0" encoding="ISO-8859-1" ?>
#     <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
#     </xsd:schema>
#     """
#     CTNChildrenTypes = [("OPCUAClient", OPCUAClient, "OPCUA Client")]
#