BListView: Impliment autoscroll

BeOS had a feature where you could click on a list and while holding
down the mouse button scroll through the list items selecting them
as you went. I implemented the same feature on Haiku.

Did the work in separate thread which makes sure that the mouse
is held down the whole time even if you leave the view.

Thanks Diver for bringing this feature to my attention.

headers/os/interface/ListView.h

@@ -189,6 +189,9 @@ private:
 			bool				_ReplaceItem(int32 index, BListItem* item);
 			void				_RescanSelection(int32 from, int32 to);
 
+			void				_DoneTracking(BPoint where);
+			void				_Track(BPoint where, uint32);
+
 private:
 			BList				fList;
 			list_view_type		fListType;