make setValue and value method virtual to allow bindings to define their own implementation of symbol like Variable symbol for exported variables
Add to variable also its type.

It is especially important for reference to function in different module which
doesn't have its signature in symbol table in XML.
Add also category of called symbol to recognize reference call
Merge branch 'allow_se_override' into y2r_fixes
properly xmlify whitespaces
fix switch body to xml
properly print to xml multi-cases
Fix type matching for functionType so return wildcard is properly matched

Problem here is that match is not simmetric as source can match against
wildcards but not vica versa. In function it is more tricky. Previous
implementation take arguments and return value same way, but that cause problem
with matching function like any(boolean) to integer(any) that actually match
(because you want pass boolean, that is fine and want anything, so also integer
is fine), but old matching alghorhytm incorrectly mark it as unmatched.
add to call information about type as it helps to recognize passing by reference
fix previous commit to write proper type string
create proper xml
fix builtin to xml to expect more then one value as in case of reduce with two inputs
Add setter for WFM::Args as other client component need to set it so their builtins WFM::Args works
unify namespace attribute in XML
Use proper name of namespace including System:: prefix as in ycp is also written System::local_ns::method
add more logging related to changing system namespaces
fix multiple import of same namespace
Implement unregister of system functions

It is needed as due to Y2Namespace documentation after createFunction caller
can immediatelly destroy Y2Function, but Y2SystemNamespace try to switch it even
if it is destroyed. Such action lead to segfault.
fix memory leaks in SystemFunctions
Fix Stdio Function to allow call of function without specified type

This is needed for components and bindings that do not require explicit type
of function paramaters