example: Split the header bars with styling

Draws a border between the header bars with custom CSS styling as done
in other GNOME applications. Also drop the now useless GtkSeparator.
1 job for css-border-split in 2 minutes and 48 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #887
librem5
build-debian

00:02:48