Merge FlxG.keyboard into FlxG.keys

Reasons for this change were discussed [here](https://github.com/HaxeFlixel/flixel/commit/273d2272c3d41c27464deefa03d96811cadfd9e7#commitcomment-5112444).

Obviously this fails travis, but I have the necessary fixes for flixel-addons and flixel-demos locally and ready to push once this is merged. It's a simple find and replace.

I do feel a bit bad for our users, since I created `FlxG.keyboard` for 3.0.0 in the first place and am now kind of reverting it again for 3.1.0, but I feel like it's for the better of the API.

I also added `@:noCompletion` to a few functions in `FlxKeyboard` to hide them from completion - that should make the API easier to use as those functions are not relevant to the user, but still need to be public / accesible (I guess we could also use `@:allow` and make them `private`).