Make symbols explicitly public, and implicitly private

Fixes GNOME/libhandy#177
11 jobs for symbol-visibility in 19 minutes and 51 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed api-visibility #830894

00:00:33

passed build-debian-clang #830896

00:04:33

passed build-debian-gcc #830895

00:03:36

passed build-fedora-gcc #830897

00:10:48

passed build-flatpak #830898
flatpak

00:03:23

passed build-flatpak-glade #830899
flatpak

00:07:09

 
  Test+Docs
passed build-gtkdoc #830900

00:03:08

passed unit-test #830901

00:03:12

44.9%
 
  Package
passed package-debian-buster #830902

00:03:16

 
  Test Package
passed autopkgtest-debian-buster-package #830903

00:02:33

passed lintian-debian-buster-package #830904

00:01:27