UI material z-offset override

[ickshonpe]

Objective

In #20237 the swatch implementation uses a child node to display the foreground color because UI materials have a higher local z offset than everything else.

Solution

Add a stack_z_offset function to the UiMaterial trait that can be overriden to set a custom z-offset.

[viridia]

You know, I was tempted to say that this change isn't necessary (for me anyway) since I worked around the issue (and might solve it a different way post-BSN). But given how simple the fix is, I don't see any reason not to approve.