scrolledwindow: Drop the decel friction on touchscreens

On touchpads, swiping multiple times on a long document is comfortable enough, and we can't stop the kinetic scrolling by simply touching the touchpad, so a strong deceleration friction is wanted.

On touchscreens though, swiping many times is tiring and can induce muscular fatigue, and the scrolling can be stopped by simply touching the screen, so it's better to have no friction and to let the inertia do the scrolling for a long time.

This drops the inertial scrolling on touchscreens but not on touchpads to make the formers more comfortable to use.