* parser/expr_evaluator: expression evaluator using Reverse Polish Notation

  * parser/expr_evaluator: expression evaluator using Reverse Polish Notation
    (a bit "C-style-coded" at the moment ;)
  * utils/var_bool.*: added true and false boolean variables, and the "OR"
    composite variable
  * src/var_manager.*: support for anonymous variables
  * parser/interpreter.cpp: use the RPN evaluator to resolve boolean
    expressions in the XML. Any well-parenthesized expression using "not",
    "or", "and", "true", "false" and boolean variables should be understood
    (not much tested yet).