Scroll speed configuration suggestions

GdkScrollUnit and GtkEventControllerScroll clients can now make a difference between mouse scroll events and touchpad/trackpoint scroll events thanks to the GdkScrollUnit field of scroll events introduced several months ago.

The problem is that users still find that the actual GtkScrolledWindow scrolls too fast while others find the actual speed confortable.

We need a way to configure the scroll speed, at least for GtkScrolledWindow which is the most present scrollable widgets on GTK applications in general.

I think that tweaking the scroll deltas magnitude at the compositor level is not a good idea because it can interfere with other scroll events use cases that are not concerned here (wheel clicks commands, scrollable comboboxes, zoomable pictures…).

So I propose to create 3 new GtkSettings keys, to be able to configure the scroll speed for mouse, touchpad and trackpoint. GtkScrolledWindow will read these settings and adapt the scroll speed accordingly. There are other custom scrollable widgets in apps like gnome-console, gnome-web… These apps can also read the same GtkSettings keys to propose a consistent experience.

The proposed MR is !4672.

This is exactly the same principle as SPI_GETWHEELSCROLLLINES in Windows but with 1 setting per scroll "source".

Thanks for your feedbacks.

Cf #3631 (closed) #1308 (closed) #1641 mutter!1840 gnome-control-center#379

Cc @carlosg