expose the keyBinding functionality

I'm not 100% sure this pull request moves the dial forward, but here's what I have. I've exposed the KeyBinding class, and also created a KeyboardHandler class. I created a ctor for _KeyboardHandlerProxy similar to the one for _ThemeProxy.

This pull request is not functional, but perhaps something will jump out at you. When calling `editor.keyBinding` I get a strange:

    TypeError: Converting circular structure to JSON

message.