Indexed property categories for x-to-many related instance properties

[grigasp]

In cases when we have multiple related instances to load properties from, we display them as struct arrays where each struct represents a related instance:

That doesn't work well if we want a custom renderer for the related properties or nested related properties.

A possible solution is, instead of creating a struct array, create an indexed category list (e.g. "Related Class [1]", "Related Class [2]", etc.). We might also want to consider using related instance label rather than related class label.

Repro:

1. Open this
2. Select any element in the viewport
3. Check the property grid at the bottom right corner