PopupMenu.PopupBaseMenuItem uses the natural width of its children to
set its own minimum and natural widths. We don't want that because
some of our children can be too wide. So, we override it.
We also set the span and expand parameters of the label to -1 and true
respectively. Otherwise we won't get to see the "...".
https://bugzilla.gnome.org/show_bug.cgi?id=693282
b9fba54b9f