Fix a crash with steppers
Fix a crash with steppers

The introduction of the trough node was not properly carried
into the code constructing stepper nodes, and was causing
assertion failures there. This was only showing up on Windows,
since Adwaita and HighContrast don't have steppers.

grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita