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This is generally expected of containers where it makes sense.

Implement request modes fully.

We expand the center child first, but only as far
as we can keep it centered.

Make it possible to fill the slots by using the
type attribute on child nodes. This is necessary
since GtkCenterBox does not derive from GtkContainer.

We prefer to give the center widget its natural size,
and we center it as long as possible.

Any slot may be unfilled, not just the center one.

It provides functionality that GtkBox used to have,
and is generally useful.

The center widget in GtkBox was only introduced to use it in
GtkActionBar. However, the implementation there is much more complex
than it needs to be, so move the center widget into GtkActionBar instead
and later remove it from GtkBox.

Its functionality has been subsumed in GtkBox.

https://bugzilla.gnome.org/show_bug.cgi?id=723929

This is the same fix that was applied to GtkHeaderBar
recently. Adding a child widget should not make it
visible as a side-effect.

No need to construct a box that gets thrown away later.
Plus, this fixes the defaultvalue test for GtkActionBar.

https://bugzilla.gnome.org/show_bug.cgi?id=721665

A widget intended to offer contextual actions for a given view.
It allows packing children into the start or end as well as offering
a single centered child box.

https://bugzilla.gnome.org/show_bug.cgi?id=721665