Update Interaction.Key to use dispatchers.

Created `Dispatcher.Keypress`, which calls callbacks when the user is moused over a target element.

`Interaction.Key` now uses `Dispatcher.Keypress`, and can take callbacks to be called on any keyCode. This change allows multiple `Interaction.Key`s to be attached to the same component.

API changes:
* `Interaction.Key.callback()` was removed in favor of `Interaction.Key.on()`, which allows the user to specify a key code to listen for.
* `Interaction.Key`'s constructor no longer takes in a key code.

Partly addresses #837 and #1209.
True, but with maps, most implementations double the keySpace once the keySpace limit is reached, which results in O(n) additions in the worst case scenario (its asymptotic so no big deal)

That said, I'm not pushing this too hard unless we can think of a reason to use one over the other