get rid of double scrolling on mouse wheel

if your mouse is over the one of vellum's scrollable sections
and you use your mouse wheel (or touchpad, etc.) to scroll
you no longer start scrolling the window when the pane reaches the top/bottom

up/down keys still have double scrolling behavior,
and you can still click the up down arrows on either scroll bar
for OS's that have that (i.e. most except macs)
add comment to double scrolling fix
Merge pull request #283 from dimagi/no-double-scrolling

get rid of double scrolling on mouse wheel