Provide basic structure for keyboard shortcuts

Summary: Implements a simple infrastructure for keyboard shortcuts, see T184, and a "help" shortcut.

There's a lot of room for refinement here but I think it basically works. Each shortcut can also provide a "tooltip" handler which allows it to show help when the alt/option key is held down.

Test Plan: Pressed "?" and got help. Pressed "?" in various contexts where it should not activate (modifier keys, text input focused) and didn't get help.

Reviewers: aran, tuomaspelkonen, jungejason

CC: moskov

Differential Revision: 362