main-toolbar: Use a full-height, splitting separator

Previously we used a separator in the middle of the main toolbar and extended the width of the HeaderBar in order to
special case for Ambiance.

Ambiance is no longer the default Ubuntu theme, and sticking
with GNOME design patterns should be prioritized over
special casing with a theme.

Fixes #389
5 jobs for <span class="ref-name">wip/christopherdavis/full-height-separator</span> in 26 minutes and 48 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #276588
fedora

00:12:30

passed #276589
ubuntu

00:14:15

 
  Test
passed #276590
flatpak

00:11:21

 
  Deploy
passed #276591
review

00:00:37

passed #276592
allowed to fail manual
stop_review

00:00:33