473433c93d
Commit 0b96b8a1 set margins via css, but accidentally changed the
semantics of margins for separators in popovers so that any separator
in a gtkpopover had a margin. This meant that the separators in
GtkListBoxes in popovers also had a margin around their separators, and
this is not what we want because it doesn't match separators in
listboxes not in popovers.
https://bugzilla.gnome.org/show_bug.cgi?id=760427