Nautilus: Shows free space in a floating label, like the selection count

This is a hack, an abuse of the provided extension API. It works with Nautilus 3.28.1-784b3eaab, but probably not with any other version.

With nautilus-python installed, shove FreeDiskSpaceBar.py into ~/.local/share/nautilus-python/extensions/

See https://github.com/climaxx/Nautilus-Status-Bar-Replacement/ for something that works well and sticks to supported things.