Function to purge unused type-registered property data

Blender stores data for properties that addons register on types, and this data persists (of course by design) even if those properties are removed from the addon, or the addon is removed entirely.

Over time this data builds up and bloats files.

Short of reading blender files and rewriting them, I'm not aware of a method to remove this 'dangling data' from the python side.

However it does seem something that should be feasible from c++ and I think would be a very nice to have :)