This directory contains the code for the "top level" GTK+ UI, which is
responsible for organizing the various views that aptitude presents to
the user.

The design provides a simple "model" of the structure of the
top-level, which acts as a go-between for the top-level UI and the
rest of the program.  The goal here is to allow us to tweak the
top-level design without impacting the rest of the code too much.