welcome: Drop HdyWindowHandle

See merge request GNOME/gnome-tour!40

Since we have a headerbar now and the first page is part
of the carousel, we shouldn't use HdyWindowHandle.

ui: update strings per 40

Closes #31

See merge request GNOME/gnome-tour!37

main: translate application name

See merge request GNOME/gnome-tour!39

As per glib docs: `set_application_name`: Sets a human-readable name for the application.
This name should be localized if possible, and is intended for display to the user.

assets: improved backpack shape

See merge request GNOME/gnome-tour!38

- more of an adventure tour vibe

fixes #31

Always show a headerbar

Closes #30

See merge request GNOME/gnome-tour!35

- avoid baloon duplicty

as the 'close' button is smaller, there's a weird
allocation thing when going from page 1 to page 2 for the first time

fixes #30

re-display the app in grid

Closes #28

See merge request GNOME/gnome-tour!34

fixes #28