An adaptive view switcher.
An adaptive view switcher, designed to switch between multiple views in a similar fashion than a [class@Gtk.StackSwitcher].
Depending on the available width, the view switcher can adapt from a wide mode showing the view's icon and title side by side, to a narrow mode showing the view's icon and title one on top of the other, in a more compact way. This can be controlled via the policy property.
To look good in a header bar, an `HdyViewSwitcher` requires to fill its full height. Contrary to [class@Gtk.HeaderBar], [class@HeaderBar] doesn't force a vertical alignment on its title widget, so we recommend it over [class@Gtk.HeaderBar].
`HdyViewSwitcher` has a single CSS node with name `viewswitcher`.