Refactor into two listeners, Mouse and Keyboard

[mwcraig]

Argument for: Simplifies some of the logic in DOMListener if mouse and keyboard handling are separate.

Argument against: There is much in common between the two cases (can be addressed by subclassing).

See jupyter-widgets/ipywidgets#1727 (review)

[mwcraig]

Current structure seems to work, so won't change it