Fixes II

See merge request GNOME/gnome-tour!61

Instead of doing multiple overlays, do a single overlay containing all
buttons as type=overlay children. This makes the carousel span the
window from start to end.

This allow us to simplify the changes on the UI part

This would allow us to construct the pages on the ui files later

it doesn't make much sense now, but it will do once we port the rest of widgets as well

makes iteration on the UI slower

This is in preparation for a future gtk-rs release

it's part of the sdk nowadays

style: Stop hardcoding page colors

See merge request GNOME/gnome-tour!56

# Conflicts:
#   data/resources/style.css

Remove redundant and empty style classes

See merge request GNOME/gnome-tour!59

general: Clean up warnings

See merge request GNOME/gnome-tour!58

* .page-body is indistinguishable from the default body style
* libadwaita uses .title-1, so .page-title can be swapped out
* .large-title was unused
* Other empty styles removed

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

The defaults work fine.

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)