Currently, if we #addItemProperty and then #removeItemProperty on the adapter, before saving, it will still try to save the changed property (it's not removed from the map).

If we blindly do this, we then run into concurrent access, because #updateProperty (internal API) also calls #removeItemProperty (public Vaadin API) when value is blank. Then it's too late to act upon #changedProperties so we need to mark properties for deletion "internally" as well.

Use case/story:
Only the Item is passed from the FieldFactory to the Form to the SaveDialogAction.
So this simply allows us to carry arbitrary transient properties (with arbitrary type) along with the adapter, without the intention to save them to JCR (by the time we replace that with some form execution context in 5.5).