constants: split C.bool in C.bool and C.str so:

- you can use them without caring about the input type
    - you always know the output type

src/core/constants.py

@@ -202,7 +202,14 @@ class Const(object):
 
     @classmethod
     def bool(cls, value):
-        """retour str text value for bool, or bool value for str"""
+        """@return: bool value for any type"""
+        if isinstance(value, str) or isinstance(value, unicode):  # dbus.String is unicode but not str
+            return value.lower() == cls.BOOL_TRUE
+        return bool(value)
+
+    @classmethod
+    def str(cls, value):
+        """@return: str text value for any type"""
         if isinstance(value, bool):
             return cls.BOOL_TRUE if value else cls.BOOL_FALSE
-        return value.lower() == cls.BOOL_TRUE
+        return str(value)