KeyboardHandler : add `DEFAULT` to `BINDINGS` and remove special casing ...

...of `null` in the get / set keyboardHandler functions

This CL boxes the `null` value of the default keyboard handler in the
KeyboardHandler.DEFAULT constant.  This eliminates special casing of
`null` in the getters / setters as we always box the value (including
null) in a KeyboardHandler object.

I think that this makes the API more uniform for integration into real apps; it also provides us w/ an abstraction of the DEFAULT value so that its underlying value can change (for instance, in a pure dart implementation this will likely be a non-null object).

@devoncarew any thoughts / objections here?