Add a new CogViewport class, which represents a container (e.g. a
window, output surface, GTK widget, etc.) which can display one web
view out from a group. The container can itself be observable using
its "add" and "remove" signals, tracks a "visible" view, and ensures
that only the visible view has the "visible" state flag enabled.

Use a CogViewport instead of a single web view. This will allow the
current platform plug-ins to show in their output one of many views
without needing many changes.

Note that in the future platform plug-ins should be updated to avoid
using globals and make them handle multiple viewports (e.g. windows)
as well.

Add an examples/ subdirectory, and a new simple-stack example program
which shows how to attach multiple views to the CogShell view stack
and switch among them.

Replace the single view saved in the CogWlPlatform by the viewport
being used by the shell, taking care that only the visible view gets
to update the contents of the Wayland surface and that input events
get sent to the visible view as well.

There are some locations where the visible view is assumed to be also
the focused one, which may not be true later on when multiple viewports
are supported. This compromise is okay for the time being and may need
to be revisited later on.