The GTK inspector would always target and inspect the `main_widget`
box rather than the parent widget, which became repetitive to keep
pressing the "Parent widget" button when working with accessibility,
as the parent widget contained the accessible properties and states.
GtkBoxLayout solves that by allowing us to get rid of the box
completely, which exclusively makes it inspect the widget we
actually want to target.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/587>