object: Add Gtk::make_managed<T>(args...) helper [gtkmm-3-24]
To avoid the discouraged pattern of manually writing `new` and get
tidier syntax, follow the example of std::make_(unique|shared)() by
adding a new helper function that both constructs and manages a widget
in one call. The user will not need to `new` and can user fewer parens.
auto button = Gtk::manage(new Gtk::Button("_Hi", true));
becomes
auto button = Gtk::make_managed<Gtk::Button>("_Hi", true);
Switch to make_managed() in tests and demo, albeit nowhere else (yet?).
Edited by Daniel Boles