This does not currently have UI exposed, but the GSetting is there for
those that want to enable it that way.

You can do something like:

  gsettings set org.gnome.TextEditor draw-spaces "['tab','space']"

The flags you can enable:

  * space
  * tab (\t)
  * newline (\n)
  * nbsp (non-breaking space)
  * leading (leading text on line)
  * text (only within text)
  * trailing (trailing text in line)

Fixes #150