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.

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

# See COPYING.Runtime file for copyrights details.

LogLevels = ["CRITICAL", "WARNING", "INFO", "DEBUG"]
LogLevelsCount = len(LogLevels)
LogLevelsDict = dict(zip(LogLevels, range(LogLevelsCount)))
LogLevelsDefault = LogLevelsDict["DEBUG"]