Improve keyboard support

Currently you can:

• Use keyboard shortcut to show/hide Substage. This also tries to be smart, so if you’re in another app it’ll first switch to Finder+Substage, and only on second press (if Substage is fully front/active) will it actually hide.

• Press Return to confirm, Escape to cancel/clear.

• Press up/down to cycle through command history

Areas for improvement:

• BUG: Sometimes when activating Substage, the prompt field isn’t selected?

• Add a way to jump back and forth between Substage and its attached Finder window.