preliminary GNOME 40 changes

Closes #29

See merge request GNOME/gnome-tour!33

- Fullcolor tour
- Basic CSS keyframe animations
- Use of default grey backgrounds due to headerbar

Fixes https://gitlab.gnome.org/GNOME/gnome-tour/-/issues/29

Remove metainfo file

See merge request GNOME/gnome-tour!30

Tour is not an app because it has NoDisplay=true in its desktop file.
Therefore, it should not install appstream metainfo. This could be
brought back in the future if the desktop file is made visible.

See discussion in:

https://gitlab.gnome.org/GNOME/gnome-software/-/issues/445

https://gitlab.gnome.org/GNOME/gnome-software/-/issues/445

Refactor `update_position`

See merge request GNOME/gnome-tour!26

The original implementation to determine the button opacity was clever,
but not easy to understand. The new implementation is more verbose,
but hopefully easier to get.

fixes #23

Remove anyhow and adapt paginator

See merge request GNOME/gnome-tour!25

Use default line length for cargo fmt

See merge request GNOME/gnome-tour!24

Anyhow was only used for paginator and
Option makes arguably more sense here than Result

Use the builder-pattern for various widgets

See merge request GNOME/gnome-tour!23

Fix typo in README.md

See merge request GNOME/gnome-tour!22