general: Clean up warnings

See merge request GNOME/gnome-tour!58

We've had a few warnings sticking around for unused pieces
of code, or things that needed some form of error handling.

Use libadwaita beta crate

See merge request GNOME/gnome-tour!57

paginator: Set scroll params

See merge request GNOME/gnome-tour!55

21df0c02 has lost the duration, but it was
longer than the carousel default. Reduce spring stiffness to have a longer
transition.

general: Use git gtk-rs dependencies

See merge request GNOME/gnome-tour!54

Because of an API change in libadwaita,
we need to use the latest version of
libadwaita-rs.

Temporarily bundles libadwaita, as the nightly
SDK doesn't have a new enough libadwaita because
we block the build.

(cherry picked from commit eecfa3ef)

(cherry picked from commit fe7e087e)

Port to GTK4 + libadwaita + gtk4-rs

Closes #36

See merge request GNOME/gnome-tour!53

This commit upgrades our rust dependencies and ports
Tour to libadwaita+GTK4. As part of porting we now use
GtkPicture instead of GtkImage, and AdwClamps to hold pages
in place.

This port is part of https://gitlab.gnome.org/GNOME/Initiatives/-/issues/26 and handles https://gitlab.gnome.org/GNOME/Initiatives/-/issues/32

Fixes https://gitlab.gnome.org/GNOME/gnome-tour/-/issues/36