wayland: convert the touchpad scroll unit to mouse wheel unit

Concern #3631 (closed).

This MR is the same as !3845 (closed) but it targets GTK4 and not GTK3 and have more consise comments.

In wayland sessions, mutter sends mouse wheel scroll events in actioned wheel notches degrees (multiples of 10). It sends touchpad scroll events in direct screen coordinates to scroll. Despite these 2 differents units, GDK divides all events deltas per 10 to convert these units in actioned mouse wheel notches count. It works for mouse wheel events but not for touchpad events.

The goal of this MR is to correctly handle touchpad scroll events in GDK.

The problem is more detailed in this message (it's the "second solution" that I implement here) and in code comments.

Thanks!