[plt-scheme] scrolling windows

Hi again,

Another usability report/request: in MacOS, the scrolling action (ie. with the mouse) affects the window/pane directly below the mouse (independently of which application is actually 'active'). 

DrScheme does not follow this metaphor:
- if DrScheme is not the active app, then scrolling with mouse cursor over a DrScheme window does not have any effect
- if DrScheme is the active app, but there are multiple panes (defs window, interaction window, splits) the pane that is scrolled is the one that was last clicked in, not the one that is below the cursor.

Thanks,

-- Éric