Migrate to libhandy 1

As of August 7, [a first beta release is available](https://mamot.fr/@KekunPlazas/104648142113069390).

The code to build can now be downloaded from [gnome.org](https://download.gnome.org/sources/libhandy/0.90/).

A [migration guide](https://gnome.pages.gitlab.gnome.org/libhandy/hdy-migrating-0-0-to-1.html) is available, just as [packages for Debian](https://packages.debian.org/bullseye/libhandy-1-0).

Fixes #59.