implement new c++ calling convention for object arguments

[davidnich]

currently declaring an object as a parameter in qpp code results in generated c++ code that requires the caller to dereference the private data pointer themselves - instead a ReferenceHolder<>& parameter should be passed, so that the method can take the reference if needed, but by default the referenced argument will be dereferenced automatically, thereby eliminating the chance for a memory / reference leak

this is a "design bug"