Expanders are usually drawn as little triangles and unfortunately
do not support rotated drawing modes. So a hack is applied (see
gtk_tool_item_group_header_expose_event_cb for details) when
drawing a GtkToolItemGroup's header for horizontal GtkToolShells,
forcing the triangle to point in the right direction. Except we
don't draw expanders as triangles on Windows. Usually, expanders
are represented as "+" and "-". It sucks for "+" to become "-" and
the inverse when we don't want to, so reverse the hack here.
7cfefd6e98