In GTK3, GtkBox, GtkScrollbar and GtkSeparator are all single classes
with a GtkOrientation parameter, whereas in GTK2 the horizontal and
vertical versions were trivial subclasses of them. Hence, the old
constructors are now deprecated, but _only_ the constructors are
affected, since after constructing one you always used methods of the
superclass on it anyway.
Rather than faff about with an ifdef at every call site, I've just put
some wrapper macros in gtkcompat.h to make it easy to keep this code
similar between all supported GTK versions.
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