This example contains a simple "hello world" program.

A more elaborate program that demonstrates advanced Gtk 3 features is
not available here, because
  - derived GObject classes have a number of pain points with gtkmm
    (see <https://thinkingeek.com/2023/02/04/writing-gobjects-in-cpp/>),
  - widget templates are not supported in gtkmm (see
    <https://github.com/rofirrim/libadwaitamm/blob/main/demo/templatewidget.h>).
