logging: Replace env_logger_pretty with env_logger

See merge request GNOME/gnome-tour!73

The env var G_MESSAGES_DEBUG is supported by tools like Builder and by
most of GNOME Core apps. So also adding support here.

Over time env_logger also got pretty output and env_logger_pretty
depends on an (outdated) version of env_logger anyways.

icons: Make navigation arrows direction-neutral

See merge request GNOME/gnome-tour!72

This corrects arrow directions in RTL.

window: Update strings

See merge request GNOME/gnome-tour!71

General polish on the words.

appdata: Improve appdata for AppStream 1.0

See merge request GNOME/gnome-tour!70

- Add the `<developer><name>` tag
- Mark the `<developer_name>` tag as deprecated
- Use appstreamcli to validate appdata
- Add vcs-browser and the translate URLs
- Update the donation URL

Update deps

See merge request GNOME/gnome-tour!69

paginator: Use AdwToolbarView, subclass AdwBin

See merge request GNOME/gnome-tour!68

* GtkBox isn't meant to be subclassed, and apps should not do it.
Instead they should subclass AdwBin or GtkWidget depending on their
use case.

* The main reason we were using GtkBox was to contain our header
and our content. Now we use AdwToolbarView for that purpose.