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
