Various fixes to mouse wheel scrolling:
* Now that WebCore uses floating-point scroll deltas, eliminate complicated carryover code and just use simple floating-point arithmetic when calculating scroll delta.
* Now that WebCore supports scrolling by page, plumb this instead of using a hacky "10 times the normal scroll amount" constant.
* Don't pretend shift was down when it wasn't (e.g. WM_MOUSEHWHEEL).
* Use SPI_GETWHEELSCROLLCHARS for horizontal scrolling, per MSDN.
* Fix horizontal scrolling to be "scroll down to go right" as the comment said (behavior was backwards)
* Clean up code.
* Reorder Mac/Linux code to match Windows code ordering.