WIP Gtk4/Libadwaita Pixelfed client

Now it loads real data from local json file (in this video, public timeline of posts tagged #shotonpinephone from mastodon.social ), with some code borrowed from [Tootle](https://apps.gnome.org/it/app/com.github.bleakgrey.tootle/) (thanks! :D) Avatars and images loaded when needed and asynchronously (no local cache, yet)