Implement support for the XML property lists that are the newer version
of the ASCII ones. There are a few things that still need to be done,
but all of the basics are there

Next things
- Allow collapsing a property list into a series of objects
- Serialize both the property list and flattened objects to XML

Yeah... not a great commit message. t.b.h, I could maybe've split the
commit into more parts; but that would be quite a lot off effort and
would have a pretty decent chance of at least one of the commits leaving
the repository in a non-working state.

For the future, will want to try and commit more often so there aren't
these mega-commits where it's just "a whole bunch of stuff changed"