GtkWindow: some min/nat size corrections.

GtkWindow: some min/nat size corrections.

Don't add the container border to the title request size; it
is only used for the child widget.

Don't call gtk_widget_get_preferred_width_for_height() for
the title bar with an unrelated height and subtract the title
bar height before querying the child widget width.

Guard against negative size requests after substracting the
borders/shadows and the title bar.

https://bugzilla.gnome.org/show_bug.cgi?id=751341