gui tree view improvements

[Aeva]

• [x] relative paths in tree view
• [ ] multi-nesting of directories in tree view not actually good ux
• [x] tree view icons (folder vs level)

---

For reference with the tree view stuff:

• http://python-gtk-3-tutorial.readthedocs.io/en/latest/treeview.html
• https://developer.gnome.org/gnome-devel-demos/stable/treeview_cellrenderertoggle.py.html.en
• http://arstechnica.com/information-technology/2009/08/howto-an-introduction-to-gtk-treeviews-and-autocompletion/