RFC: Reference validation

[akphi]

One big concern we have is when we delete something, other parts of the app still keeps the reference to it. For example: class properties being referred to in class mappings, etc. Right now our strategy after deleting some element is quite inconsistent:

• For deleting element, we do a full rebuild/recompile
• For deleting sub-element, we do nothing

The former is a bit aggressive if we need to do this for every change. That being said, it might be useful to tell user when references are broken/invalid.

Implementation Plan

• Add a flag isDeleted for places we want: element, property (basically for each type of reference)
• Add a computed property to each reference (potentially for reference we can do a method isValid() always returning true by default, and we can override for each as we start supporting more and more things) -> this is good for graph validation checks in general. See RFC: Rework graph feedback #262

[akphi]

Closing in favor of #1168