GSound depends on libcanberra, but it's an indirect dependency, so
if we use GSound, we don't need to use libcanberra.
GSound is available on Ubuntu 16.04, so we can switch to it on
most systems. The libcanberra backend is still supported for now,
though it might be removed later.
Since GSound doesn't have a GTK+ dependency, we can use it in the
GTK4 backend. (libcanberra doesn't support GTK4.)
