Add a helper macro for setting standard button callbacks in a dialog.

Yes, it's a macro, but it can reduce the copy-paste factor of a
pre_show() function quite a lot when there are many buttons.

src/gui/dialogs/helper.hpp

@@ -33,6 +33,12 @@ void dialog_callback(twidget* caller)
 	(dialog->*fptr)(*window);
 }
 
+#define GUI2_EASY_BUTTON_CALLBACK(ID, MY_TYPE) \
+	tbutton* ID##_btn = dynamic_cast<tbutton*>(window.find_widget(#ID, false)); \
+	VALIDATE(ID##_btn, missing_widget(#ID)); \
+	ID##_btn->set_callback_mouse_left_click(dialog_callback<MY_TYPE, \
+		&MY_TYPE::ID##_button_callback>);
+
 } // namespace gui2
 
 #endif