`GtkEventControllerFocus` is an event controller to keep track of keyboard focus.
The event controller offers [signal@Gtk.EventControllerFocus:GtkEventControllerFocus:enter] and [
signal@Gtk.EventControllerFocus:GtkEventControllerFocus:leave] signals, as well as [
property@Gtk.EventControllerFocus:is-focus] and [property@Gtk.EventControllerFocus:contains-focus] properties which are updated to reflect
focus changes inside the widget hierarchy that is rooted at the controllers widget.