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.