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.

.project

.svghmithumbs

.directory
.pytest_cache
.cache
.coverage_full

syntax: regexp
^tests/.*/build$
^exemples/.*/build$
^.idea/.*
syntax: regexp
^.*\.pyc$
syntax: regexp
^.*~$
syntax: regexp
^.*\.swp$

bug_report.*\.txt
i18n/.*.new$
revision

doc/_build
doc/locale