PySide Type Hinting

Shortly after the TP, people began to ask for type hinting as IDE support.
Since the signature extension exists now for quite a while, it makes sense
to interface somehow to IDEs.

Some IDEs can use the __signature__ property directly, which is most elegant
and provides more information by introspection.

For all other IDEs, Python Stub Files (.pyi) are generated during execution of
the setup.py script.

This task has low priority since it is itself debatable to be just nice-to-have,
but it puts a high impact on the correctness of several modules which are
connected to the signature extension.