assets: Software grid alignment

See merge request GNOME/gnome-tour!15

- align Software icon
- adjust grid position and alignment
- replace some app icons

See https://gitlab.gnome.org/GNOME/gnome-tour/-/issues/17

We no longer need to manually keep it in sync with the ci.

Fix minor typo: tying -> typing

See merge request GNOME/gnome-tour!14

Update labels

Closes #18

See merge request GNOME/gnome-tour!12

assets: tweak page #6

See merge request GNOME/gnome-tour!13

- Add dropshadow for the Software icon, replace Banner viewer with
  Foliate

Based on design review https://gitlab.gnome.org/GNOME/gnome-tour/-/issues/17

General polish for the page text.

Fixes #18.

paginator: Update position after adding pages

See merge request GNOME/gnome-tour!10

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

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