St has the regular abstractions to handle actors that are bigger
than their parent could handle: StScrollable, StScrollView, and
StAdjustment.
However, the only StScrollable implementation available currently
is StBoxLayout, which forces a ClutterBoxLayout as the layout
manager (and relies on it not being unset).
Introduce StViewport, which is a minimal StScrollable implementation
that doesn't rely on any specific layout manager.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/929
