TODO: A way to see the path of the currently selected item in any grouplike
      element. Each item in the path should be a button that, when pressed,
      makes the MAIN grouplike view navigate there.
      (Done!)

TODO: A "shuffle queue" button!
      (Half-way done -- the functionality is there, press "S", but there's no
      button for it yet.)

TODO: A "clear queue" button.

TODO: A way to jump to an item with a particular name. Probably press "/".
      It'd be nice if the closest match got highlighted while you were typing.

TODO: "Queue to play next"... maybe also a cursor in the queue list, which
      would tell after what track to place newly-queued items?

TODO: Scroll to the selected track once it starts playing when selected from
      the queue.

TODO: process.on('SIGWINCH', () => ...) -- detect terminal resize!!
      (Done!)

TODO: Pressing enter in the queue seems to not be doing the right thing?
      It should NOT move the selected item anywhere in the queue; it should
      just select and play that track.
      (Done!)

TODO: iTunes downloader - test this.

TODO: Warn if no mkfifo (means controls won't work).

TODO: file:// support for crawl-local.
      (Done!)

TODO: Pass YouTube playlist or http://.../playlist.json-like URLs to use them
      as playlists (crawl automatically).
      (Done!)

TODO: There's some weird glitch where, if downloaderArg is missing (=== ""),
      it'll play.. something by Jake Chudnow?? -- Okay, looks like it's using
      ~/.mtui/downloads/<locallink, etc>/ as the directory for where it would
      put the download file (because it's .../localink/ + encode(dlArg) and
      dlArg is empty). The way the cache works is that it checks if there is
      already a file in that directory, and there IS: a directory for another
      track download! But the cache doesn't know this; it just thinks that
      directory is the MP3 file (or whatever). So it returns it. MPV works fine
      if you pass it a directory that contains music files, so in my case,
      72_food (by Jake Chudnow) plays. (That's the first thing returned by
      readdir, I suppose.)
      (Done!)

TODO: Mouse support, obviously.

TODO: Ctrl-O'ing a playlist sets the left-pane's selected index to the second
      item, for some reason. (Regardless of what you had selected before..)

TODO: "Open in new tab" in the Open prompt.

TODO: Inspect the inevitable memory issues that are absolutely 100% sure to
      become a problem with opening playlists on the fly.

TODO: Canceling the "enter a playlist source" dialog also stops the current song.

TODO: Move TabberElement to tui-lib, at some point.

TODO: Make PathElements selectable (again).