mousewheel functionality we had in earlier releases in a similar way, rewritten
for handlers. (The benefit of handlers in this case is that handlers are
write-once, use many, so the next mousewheel thing we get will *also* be able
to use this code.)
git-svn-id: http://svn.openlayers.org/trunk/openlayers@2877 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf