New: support line-by-line scrolling with old scroll wheel devices in Firefox.

Without this change, using a traditional scroll wheel mouse with discrete steps would result in very slow scrolling in Firefox.

With this fix we handle these events like in Cocoa: as "[im]preciseScrollingDeltas", which `CPScrollView` in turn knows to apply the configurable line scroll amount for. For backwards compatibility, [event deltaX] and [event deltaY] are premultiplied with a suitable constant, while [event scrollingDeltaX] and [event scrollingDeltaY] show the true values needed.

Browsers other than Firefox seem to always send pixel scrolling information even for old style scrolling devices, at least on the Mac, so are not affected by this change.

Fixes #2013.

AppKit/CPScrollView.j

@@ -1403,7 +1403,16 @@ Notifies the delegate when the scroll view has finished scrolling.
     if (![_horizontalScroller isHidden] || ![_verticalScroller isHidden])
         _timerScrollersHide = [CPTimer scheduledTimerWithTimeInterval:CPScrollViewFadeOutTime target:self selector:@selector(_hideScrollers:) userInfo:nil repeats:NO];
 
-    [self _respondToScrollWheelEventWithDeltaX:[anEvent deltaX] deltaY:[anEvent deltaY]];
+    var deltaX = [anEvent scrollingDeltaX],
+        deltaY = [anEvent scrollingDeltaY];
+
+    if (![anEvent hasPreciseScrollingDeltas])
+    {
+        deltaX *= (_horizontalLineScroll || 1.0);
+        deltaY *= (_verticalLineScroll || 1.0);
+    }
+
+    [self _respondToScrollWheelEventWithDeltaX:deltaX deltaY:deltaY];
 }
 
 - (void)scrollPageUp:(id)sender