popupMenu: Remove excess padding from separators

Currently separators get all the padding from regular menu items,
which is excessive for non-interactive elements.

Shuffle style classes around a bit to allow overriding the normal
padding for separators.

GNOME/gnome-shell!1004
6 jobs for !1004 with leaner-separators in 5 minutes and 4 seconds (queued for 3 seconds)
detached
Status Job ID Name Coverage
  Review
passed #592180
check_commit_log

00:00:23

passed #592182
eslint

00:00:57

passed #592181
js_check

00:00:30

 
  Build
passed #592183
build

00:02:32

 
  Test
passed #592184
test

00:01:33

passed #592185
test-pot

00:00:49