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.

The issue names no files, tests, or entry points. Start by defining the complete shortcut plan and the discovery UX described in the issue, using Mailbox's Command-key overlay as the reference. Done requires an agreed plan and UX direction; implementation is explicitly deferred.

Written by the indexing model from the issue text.