giscanner: Add support for static virtual functions

We have a number of virtual functions in both interfaces and classes that are de-facto static methods that can be implemented to be used as factory or other similar behavior.

These are currently not supported by bindings even though it's quite simple to support them and many languages (including JS and python) support static methods.

Added some test implementation to gimarshallingtests and some extra vfuncs to the Regress header file to test that the generated artifacts respect the expectations.