Handle foreign data in U. F. O..

This uses the PyTuple, which has no matching entity in X. M. L., as a special container for unrecognized data in python_persistent, allowing FontForge to round-trip that data in the top-level lib.plist and in the lib data for individual glifs.

@DavidRaymond, we would need to add the splinechar to the arguments for XMLEntryToPython and LibToPython in order to do it completely, and that would probably undesirably clutter the code. Parsing of 

We could add a verbose U. F. O. import option that outputs the name of each glyph as it's read so as to make attribution of errors easier.