wizard: Introduce the "Initial Setup" page
This page is shown the first time a new box is created, describing what can be achieved with the assistant.
It detects the first run from a gsetting: $ gsettings get org.gnome.boxes first-run
This is based on the wizard mockups available at https://gitlab.gnome.org/Community/Design/app-mockups/raw/87e07fbc4/boxes/boxes-newbox-assistant.png