Renamed the horizontalScrollbar() and verticalScrollbar() member functions to horizontalScrollBar() and verticalScrollBar(), respectively.

The virtual moveContents() member function, declared in FXScrollArea, is now protected instead of public. This change was made to prevent programs from erroneously calling moveContents() directly to shift the window's contents. The correct approach is for client programs to call the setPosition() function, which verifies its inputs and in turn calls moveContents() to adjust the scroll area's contents.