Add Glade signal handlers example if applicable

[adnan449]

Glade makes it possible to connect to functions to emitted signals. In python they usually create a class called Handler and define methods there. And in Glade the author connects those methods names to appropriate signals: https://python-gtk-3-tutorial.readthedocs.io/en/latest/builder.html#example

Does Gtk-rs offer any such solutions? Thank you.

[sdroege]

examples/src/bin/builder_signal.rs

Lines 31 to 44 in 5d38aff

	builder.connect_signals(move |_, handler_name| {
	// This is the one-time callback to register signals.
	// Here we map each handler name to its handler.
	if handler_name == "button1_clicked" {
	// Return the signal handler.
	Box::new(clone!(@weak dialog => @default-return None, move |_| {
	dialog.show_all();
	None
	}))
	} else {
	panic!("Unknown handler name {}", handler_name)
	}
	});

is an example of that. It's not very convenient yet, see gtk-rs/gtk3-rs#128