need to expose API version

Some nautilus-python extensions could easily be made compatible with either the old nautilus-python (<< 43) or the new 4.0 series compatible with nautilus 43.

Could you provide a way for developers to look up the nautilus-python API version? I didn't see anything like this when I browsed the API docs.

Then developers could just add a conditional based on the runtime version of nautilus-python.