Part of Ink's focus on accessibility is formalizing support for keyboard

shortcuts. Components will be able to easily list keyboard shortcuts

that trigger an element, providing both a visual indicator and ARIA

information for screen readers.
Keyboard shortcuts will be registered in the new

KeyboardShortcutRegistry model, which takes a key, modifier flags, and

a callback handler, and registers it for quick access.
A corresponding KeyboardShortcutRegistryView can be attached to an

existing element and a registry model to invoke the handlers when those

keys are pressed.
Once attached, a registry view can be found on any ancestor DOM element

by using KeyboardShortcutRegistryView.findNearestRegistryView.

Similarly, a nearest registry can be found using

findNearestRegistry().. This makes it easy for components to

automatically register with the correct registry.
The plan is to eventually let the view pop up a dialog when ? is

pressed, to show the available keyboard shortcuts. This has been a

long-standing plan for Review Board, and with these new foundational

classes, we'll be able to offer this.

Unit tests pass.