4.2 work

[migueldeicaza]

I have started a 4.2 branch where work that relies on 4.2 can start.

Issue #257 is tracking the assort nice-to-haves for the 4.2 release. I have landed already the API changes (as of today, will need to update again, closer to the final release), and added support for Godot Callables constructed from a Swift function, which should come in handy for nicer signal hookups.