removed port-config from prefs
oops, sys.path config was still looking in `etc`
scattered the export-management code

- the *Session classes now live in lib.io (since they’re all wrappers
for the objc classes in the lib)
- the context managers moved to image.py (since they need a _ctx
reference)
- graphics.py now handles the export() command directly
slimmed down lib.io