Useful method for getting Buffer's dir.
Out with f*ing JavaFX tabs.

The focus issues were super annoying and it's not really that useful to see
what buffers you have open across the top of the view. If you're even remotely
hardcore there will be too many anyway.

Eventually I'd like to deemphasize the notion of "open" buffers anyway, and
move to a "go to this file in this project" intent, which won't care if the
file is open yet or not.
Factored completers into helper; wired up find-file.

There are rough edges, but we're inching ever closer to being a real editor.
Focus fiddling, better newBuffer handling.

We now properly revisit an already open buffer if you try to create a buffer
with an already open file, and we create a blank buffer if you try to create a
buffer for a non-existent file.
File completion improvements.

- Don't descend into a directory if there are also files that start with the
  directory name (this is chiefly apparent with '.').
- Better handle the edge case of completing / (on Unixen).
Manage layout manually.

This allows us to manage the z-order of the minibuffer versus the main buffer
so that popups have proper z-order. Unfortunately BorderPane seems to freak out
if you call Node.toFront on any of its children.
Start with a scratch; don't require 1+ files.