It's ok. We can leave this signal here (it's the first signal on ISettings). Anyway I'm trying to avoid them (or use them only on widgets). I remember that I've used them on app::App class, but now I think that an AppObserver class would be much better. In this case, an SettingsObserver could be better (also a lot of signals in App class should be overridable member functions in SettingsObserver, e.g. PenSize/AngleBefore/AfterChange, CurrentToolChange).

It is up to you. If you have time to create an SettingsObserver you can go for it. If not, we can merge the pull request as it's.