demo: Adjust manual titles to match HdyStatusPage

Use style classes rather than label attributes, make sure they wrap with
word-char mode and are centered.

Make the "Return to the preferences" buttons in preferences centered to
avoid stretching them with larger titles.
11 jobs for drop-attributes in 12 minutes and 58 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed api-visibility #1077936

00:00:13

passed build-debian-clang #1077938

00:03:09

passed build-debian-gcc #1077937

00:02:36

passed build-fedora-gcc #1077939

00:03:53

passed build-flatpak #1077940
flatpak

00:01:48

passed build-flatpak-glade #1077941
flatpak

00:03:09

 
  Test+Docs
passed build-gtkdoc #1077942

00:03:12

passed unit-test #1077943

00:03:09

46.3%
 
  Package
passed package-debian-buster #1077944

00:03:14

 
  Test Package
passed autopkgtest-debian-buster-package #1077945

00:02:37

passed lintian-debian-buster-package #1077946

00:00:47