GUI2/Toggle Button: fire NOTIFY_MODIFIED when setting value

This matches other widget such as the toggle_panel.

src/gui/widgets/toggle_button.cpp

@@ -121,7 +121,15 @@ void toggle_button::set_value(const unsigned selected)
 
 	// Check for get_window() is here to prevent the callback from
 	// being called when the initial value is set.
-	if(callback_state_change_ && get_window() != nullptr) {
+	//
+	// TODO: see if this is necessary with the signal handler.
+	if(!get_window()) {
+		return;
+	}
+
+	fire(event::NOTIFY_MODIFIED, *this, nullptr);
+
+	if(callback_state_change_) {
 		callback_state_change_(*this);
 	}
 }