themes: Give margins to title view switchers

This gives margin to the view switcher within the HdyTitleViewSwitcher
itself but not the whole widget, so that the window title can still take
all the available space.

Fixes GNOME/libhandy#149
11 jobs for view-switcher-title-margins in 34 minutes and 44 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed api-visibility #842528

00:02:40

passed build-debian-clang #842530

00:03:56

passed build-debian-gcc #842529

00:03:01

passed build-fedora-gcc #842531

00:12:38

passed build-flatpak #842532
flatpak

00:02:22

passed build-flatpak-glade #842533
flatpak

00:05:33

 
  Test+Docs
passed build-gtkdoc #842534

00:12:09

passed unit-test #842535

00:02:38

44.3%
 
  Package
passed package-debian-buster #842536

00:07:15

 
  Test Package
passed autopkgtest-debian-buster-package #842537

00:02:37

passed lintian-debian-buster-package #842538

00:02:23