Fix the touchpad scroll speed
This merge request is the continuation of !3766 (closed) and concerns #3631 (closed). I have tried to explain all that I've done in the code comments. Normally, the coding style is better than in the previous merge request.
The scroll source is not inspected by the GTK side but by the GDK side here. I want to include this patch in GTK 3 first because it concern more apps. If it's accepted, I will submit an other request for GTK 4. Tell me if my explanation comments are not clear, I'm not a native english speaker. Thanks!