build: Use gnome.post_install

Also requires meson 0.57.0.
Otherwise the post install script will fail if GTK 3 is not installed.

Thanks for reviewing this! 😃
Related:

• https://github.com/mesonbuild/meson/blob/master/docs/markdown/Gnome-module.md#gnomepost_install
• gnome-text-editor@0a9bca3e
• https://github.com/NixOS/nixpkgs/pull/160343