Always keep the order:
- [value]
- [marks.top]
- [marks.bottom]
- trough
Which makes sense given the rendering order. Slider should be drawn
after the marks.
Makes it possible to simply remove the custom snapshot implementations
in scale and range. And Adwaita does not depend on the node order
anyway.
Add :dir(ltr) where expected, i.e. everywhere we now have a widget but
had a gadget before.
Also, fix the expected output to expect mark subnodes in the order
specified in the GtkScale does, i.e.
├── mark
├── [label]
╰── indicator
for marks at the top of the scale and
├── mark
├── indicator
╰── [label]
For marks at the bottom of the scale.
