Adding focus / unfocus from terminals

[toumorokoshi]

There's a neat feature in terminal-plus that allows you to focus and unfocus from the open terminal. This PR is a first draft at adding similar functionality to terminal-tab.

It's really nice from a workflow standpoint, since I can quickly open and focus a tab on demand.

Open to any other suggestions on implementing this functionality as well. If this approach looks good, I can write some tests.

[jsmecham]

I like the idea of this very much. Can we work on polishing this up?

[toumorokoshi]

@jsmecham I merged in changes from master, cleaned a few places, and added unit tests.

I didn't add any default keybindings for focus / unfocus: it's hard to find keybindings that work, and I figure we will have more options once the refactoring of the way the bindings works happens.

Also did a mini review on the code, to note my thoughts on implementation.

[calebmeyer]

Something I ran into today (I'm not sure if this PR adds/fixes it) is current terminal tab doesn't re-focus the last text editor when you type exit at a terminal.

[toumorokoshi]

Yeah, this won't fix that issue, but it's an easy add.

I'm hoping to get this PR merged in, then I can send out another one for that issue.