example: Move the separator between the header bars

When a separator is between two header bars it will take a special
style, better fitting its purpose. This moves the start header bar and
the separator following it into a box so they remain a single entity to
use in size groups, while making the separator a sibling of the header
bar to benefit from this special style.

This reveals the following bug in GTK+:
https://gitlab.gnome.org/GNOME/gtk/issues/1231
1 job for separator in 2 minutes and 10 seconds
Status Job ID Name Coverage
  Build
passed #993
librem5
build-debian

00:02:10