We don't want separators in both side of an iconic section
and use a 10px margin to separate two iconic sections.
Separators are also updated in case of dynamic insertion
( often used with menu items for plugins )
https://bugzilla.gnome.org/show_bug.cgi?id=738650
...unless they are labeled.
To implement this, keep track of the nesting depth of section boxes,
starting from 0 for the toplevel and the submenu ones, and only
insert unlabeled separators if the depth is at most 1.
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.
