Amount to scroll windows by when spinning the mouse wheel.
This is an alist mapping the modifier key to the amount to scroll when
the wheel is moved with the modifier key depressed.
Elements of the list have the form (MODIFIERS . AMOUNT) or just AMOUNT if
MODIFIERS is nil.
AMOUNT should be the number of lines to scroll, or nil for near full
screen. It can also be a floating point number, specifying the fraction of
a full screen to scroll. A near full screen is `next-screen-context-lines'
less than a full screen.