Draft: Don't use pyatspi2's Registry objectg; use our own

This copies the just the source code for the Registry object, and removes everything that Orca doesn't use. It ends up being pretty minimal - some further cleanup is possible, but there are bigger fish to fry.

I have not tested this yet!!! I need to get my Orca testing setup going. It's for review only, for now.

Part of #300 (closed).