Stop propagation of keydown/textInput events to prevent React handler

React's global synthetic event handler is somewhat expensive. This
prevents it from being invoked on every keystroke, saving ~1ms.
Update emissary for Emitter::emit optimization
Upgrade keymap to avoid temp objects in keystrokeForKeyboardEvent
Upgrade keybinding-resolver for spec fix
Merge pull request #3146 from atom/ns-latency

Improve cursor movement and typing latency a bit