- avoid baloon duplicty

fixes #30

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

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

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.

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

closes #19

you should be able to configure the video path at build
time by passing --video_path=absolute_path
which will be played instead of displaying the LOGO

General polish for the page text.

Fixes #18.

Otherwise it's mixed state until we swipe carousel the first time or
press Next.

Use with_label() instead of a manually created label, make the headerbar
ones translatable, add mnenonics everywhere.

Take the position and round it, make the transition a bit more responsive.

Remove redundant properties, stop setting width request. Add a size group
and put both buttons into it so that they always have the same width.

Separate next and close buttons, do a stick-to-finger crossfade on last
page.

this also glib::get_os_info which requires glib >2.64
closes #10

Would love to keep using gtk4, but we can't get it into 3.36 otherwise.

Provide a fun engaging closure to the guide.

Things left to do:

- adjust wording
- set background class to .last-page for the style to work

Fixes https://gitlab.gnome.org/GNOME/gnome-tour/issues/7