Suggestion: make scrolling change speed

[Moonyol]

Make scrolling change the speed similar to spectator, there could be an option to need to hold alt

[MattSturgeon]

Related to #48.

I played around with doing something like this a while ago. It should be viable though not a priority right now.

[MagentaRuby]

If #48 is added with this feature, I think scrolling while holding sprint should change the sprint speed relative to normal speed. And scrolling without holding sprint would change the normal speed while changing the sprint speed to keep the same ratio. Changing the two speeds in the settings would still just change the absolute values.

And if the alt option is added, I think that the default behavior should be changing speed with alt+scroll switching items instead. A setting would switch this behavior around.