The code gets rid of the GtkTreeView and replaces it with a GtkListBox.

Most of the logic is now done via GListModel subclasses.

A big change is that this new list is now tracking updates itself and
doesn't need to be manually updated. All code that used to cause rescans
or add forgotten objects to the tree has been removed.

If objects are missing from the object tree, the logic for tracking them
needs to be added.