Make GtkTreeView easier to use

[romgrk]

This includes making GValues handling easier: https://mail.gnome.org/archives/commits-list/2011-February/msg07882.html https://bugzilla.gnome.org/show_bug.cgi?id=642914

Good example of how easy we want this to be: https://python-gtk-3-tutorial.readthedocs.io/en/latest/treeview.html https://developer.gnome.org/pygtk/stable/class-gtkliststore.html

Ideally, the lines here should be hidden from the user.

Linked: #108