Overhaul MemberChange

[bddckr]

• Get rid of InspectorEditor.
• Use Undo.postprocessModifications to notice changes done.
  • Set the backing field to previousValue if Unity already changed it.
  • Call the setter, passing currentValue.
• Use Undo.undoRedoPerformed to get told about undo or redo happening.
  • Since this event doesn't tell us what actually happened (undo or redo?), nor which property it affected, we need to check that ourselves. Do so by remembering the postprocessModifications above and go through each of those SerializedPropertys to compare their serialized value (which is the current value after Unity did this undo/redo operation) against the remembered values.
  • Revert the backing field to the "previous" value.
  • Call the setter, passing the "current" value retrieved previously from the SerializedProperty.
• Replace Zinnia.ObservableListEditor by merging its functionality into this overhauled way instead.

Related.