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.

<?xml version='1.0' encoding='utf-8'?>
<BeremizRoot xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <TargetType>
    <Generic Command="make -C %(buildpath)s -f ../project_files/Makefile all BEREMIZSRC=%(src)s BEREMIZCFLAGS=%(cflags)s MD5=%(md5)s USE_BEREMIZ=1 FROM_BEREMIZ=1"/>
  </TargetType>
  <Libraries Enable_Native_Library="false" Enable_Python_Library="false"/>
</BeremizRoot>