Cache the previous field parent element when creating inspectors

[Varneon]

Current version of VInspector requires FieldParentElement attribute to be defined on every field.

Caching the previous parent element attribute would make them function more like Header attributes, also making it easier to migrate from default header attributes to field parent element attributes.

The chain of fields parented to a certain element should be able to be broken with [FieldParentElement(null)], which reverts the cached parent back to default inspector element.