Support horizontal scrolling with Shift modifier key

All Qt scrollviews implement horizontal scrolling with a scroll wheel by requiring that the Alt key is held down to scroll horizontally. The use of the Alt key to trigger this scroll behavior is unique to Qt; all other GUI toolkits and apps have since standardized on the Shift key instead. Users familiar with this convention are likely to try Shift+scroll on a Qt app, see that it doesn't work, and get mad at the app developer.

It might be worth considering switching the horizontal scrolling modifier to Shift in the interests of cross-platform usability, or making Shift+scroll work in addition to Alt+scroll