Add an optional status bar to the bottom

As seen in the original mockups:

We've apparently discussed this in a ton of threads but never had an issue for it. So this is that issue.

Things to consider:

• It must be optional. Many users won't want a status bar.
• How does it play with tabs on the bottom? See #835
• What info would a user want there?
  • Username, machine name, date/time are all easy kinda Hard, because of SSH.
  • Keep in mind that many things from the shell (child process) we won't be able to access. Things like current path, git branch, active codepage.
    • Working path might be possible with OSC 7 (in #3158), though, it'll only work for shells that set it (which won't include cmd.exe).
• How does the user specify the contents of the statusbar? Alignment of pieces of info?
• Should the user be able to add buttons to the status bar that perform actions? E.g. a "gear" button to quickly open the settings?

These are all things that should be enumerated in a spec before we review a PR.