Some UI cleanups

See merge request GNOME/gnome-tour!9

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 is necessary to load the styles, needed for HdyWindowHandle to be
draggable (yay style properties) and for HdyWindow to have proper corners.

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

There's currently no Rust SDK extension available for master.

licence: Replace with Markdown version

See merge request GNOME/gnome-tour!8

Replaced LICENSE.md with official Markdown GPL from
https://www.gnu.org/licenses/gpl-3.0.md

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