Sort indicators for columns in GtkTreeView

[Darin Adler <darin eazel com>]

Both Evolution and Nautilus display arrows to indicate sort direction in the
sorted column in their list views. (Using ETable in Evolution and a munged
GtkCList in Nautilus.)

Is there some support for this in GtkTreeView? If so, is there a good way to
make this themeable? In my opinion, it's not appropriate to draw the
scroll-bar arrow in here (which is what ETable does), because I think of
this as a sort-direction indicator, not a special kind of "arrow". But
leaving my personal biases aside, what's the plan here for GtkTreeView?

    -- Darin