Add tests for the new CangjieContext

I'm just taking the tests we had in the previous libcangjie here.

Obviously, most of them depend on not yetr re-implemented functionality,
so I'm just stubbing the out for now, to not forget about them when the
time comes.
Add a PyGI override for the CangjieContext

Just like the CangjieChar, this merely makes the API nicer to use, at
least when it comes to accessing/setting the properties.