Add keypress for mousewheel zoom in VISUAL
Background
Currently, scrolling the mousewheel while in IGV/VISUAL zooms in/out of the view, overruling the usual page scroll. This is conventient, but does not work well when there are many tracks, where scrolling the page is often necessary. The option "Alt + mousewheel" has been judged favourably by users.
Implementation
Add a requirement of holding the "Alt" button for zooming, i.e. "Alt + mousewheel" zooms in/out, "mousewheel" is the usual scroll.
Edited by Morten C. Eike