If we still want to consider scrollbars as having an independent live as
range controls, we probably want to allow them to be flippable like the
other horizontal range controls. 
Currently the "range controls" example in testgtk looks odd in RTL mode,
since everything gets flipped except for the scrollbar.
Flippability would have to be optional for scrollbars, since we don't want
them to flip if they're part of a scrolledwindow or some other scrolling
container.