Instead of using GtkMenuTracker to flatten the sections into a single
linear menu, handle the sections ourselves by nesting boxes.
Each section gets an inner and outer box. The inner box numbers its
children in the way that the tracker instructs. The outer box
containes the inner box and the separator, if appropriate.
Having the two separate boxes will allow us to change the orientation of
the inner box if we want to pack widgets horizontally within a section.
