We didn't have a way to hide children given the allocation size
available set by the parent.
This is an use case for the GtkPathBar, which will be rewrote in future
patches where we will want to use a composite widget instead of a all
custom allocation widget.

For that, implement a container which hides widgets when the allocated
size is smaller than the requested size by its children.
The code is made by Rafał Lużyński for gnome-software and is adapted to
Gtk+ standards.

Now will follow a few patches improving the code and adding support
for some features needed by the GtkPathBar.