Handle the scroll wheel changing over scrollbars.

* Extract the scrollbar change based on the mouse wheel delta into a protected
  method of BView.
* Call that method from BScrollBar's MessageReceived.

With this change it is now a bit easier to scroll horizontally around the
system by putting the mouse cursor over a horizontal scrollbar and using the
wheel.

Fixes #8631.

headers/os/interface/View.h

@@ -584,6 +584,8 @@ protected:
 
 	virtual	void				LayoutChanged();
 
+			void				ScrollWithMouseWheelDelta(BScrollBar*, float);
+
 private:
 			void				_Layout(bool force, BLayoutContext* context);
 			void				_LayoutLeft(BLayout* deleted);