GTK+: Update the titlebar related sections

Replace the Header Bar and Separators section by the Title Bar section
and the Selection Mode section.

These explain how to handle these UI elements in a simpler and better
way, taking recent improvements to GTK+ and libhandy into accounts.
1 job for <span class="ref-name">titlebar</span> in 46 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #2395
librem5
job

00:00:46