popovermenu: Reinstate section title handling

We were creating a hbox with separators around the label,
but forgot to arrange sizing to prevent the separators from
shrinking into nothingness.
Status Job ID Name Coverage
  .Pre
passed #675778
style-check-diff

00:02:57

 
  Build
passed #675779
fedora-x86_64

00:08:38

passed #675781
win32
msys2-mingw32

00:07:03

passed #675780
release-build

00:04:41

 
  Analysis
passed #675782
static-scan

00:59:41

 
  Docs
passed #675783
reference

00:20:41

 
  Flatpak
manual #675784
flatpak allowed to fail manual
flatpak-manual:demo
manual #675786
flatpak allowed to fail manual
flatpak-manual:icon-browser
manual #675785
flatpak allowed to fail manual
flatpak-manual:widget-factory