Keyboard shortcuts

We want to have first-class support for keyboard shortcuts, and we want it to be extensible.

What needs to be done:
- Work out a complete plan for what keyboard shortcuts will do what.
- Design what the UX for keyboard shortcut discovery will look like. I really like what the Mailbox app for Mac does – when you hold down the Command key, it superimposes the keyboard shortcuts directly over the UI elements that they activate. I think that would be a very cool way to do it, as opposed to a big wall of keys/actions (not easy to learn!)
- Implementation is something I'll sit down and nut out at some point.

No files, tests, or implementation entry points are named. Start by separating the requested shortcut plan, shortcut-discovery UX, and later implementation work; review the Mailbox-style overlay reference and define the complete shortcut actions and discovery experience. Done means the plan and UX design are agreed and ready to implement.

Written by the indexing model from the issue text.