Add a setting to change the vertical scrolling multiplier

[eigenvoid]

The hardcoded scrolling multiplier of 3 doesn't make much sense since most music has a time signature of n/4, so I think there should be a setting to adjust it to another value. I also think 2 would be a better default value.

Also, when playing around with the scrolling multiplier

MilkyTracker/src/tracker/sdl/SDL_Main.cpp

Line 496 in e205287

mouseWheelParams.deltaY = wheelY * 3;

I found out that commit 9c9c9c0 makes it so that scrolling doesn't work properly with values other than 3. With a value of 2, scrolling up will scroll 2 lines, but scrolling down scrolls 3 lines.

[coderofsalvation]

hm scrolling odd vs even values sounds a bit weird indeed.