[Feature Request] Add a bar/visualization of z-order/sorting

[EnjoyerofOpenSourceContent]

Validations

• I have checked for similar feature requests and could not find any.
• I have made sure this is not an already-existing feature.

Description

I and I'm sure other riggers, would appreciate a way to visualize how the numerical value in "Sorting" equals to the z-order and how the parts are arranged.

Suggested solution

Include some UI showing a bar marked with some values and the associated z-order like how Live2d does it. Or some other UI that can otherwise visualize the sorting values and the z-order.

Alternative solution

Alternatively, you could show a popup when hovering over the Sorting area telling the user that negative values = parts further up front.

I'm sure there are other ways to implement this, but these are the main ideas that come to mind right now.

Additional Context

I would also like a feature to set the sorting order with multiple parts selected all at once, but that may be outside the scope of this feature request.

[r888800009]

There are two things to consider about this feature

1. The inochi2d node will inherit the zsort of the parent node. For the this feature, which node zsort should we control and how to control it?
2. imgui has a vertical slider, but it cannot display scales, that means we need to implement another widget.