* examples/book/combobox/entry_complex/examplewindow.[cc|h]:
* examples/book/combobox/entry_text/examplewindow.[cc|h]:
Use Gtk::EventcontrollerFocus::signal_leave() to be informed when the
keyboard focus leaves the Entry widget. Use Gtk::Entry::signal_activate()
if it exists, i.e. if gtkmm >= 4.8.0.