Add support for removed schema elements

[StefanApfel-Bentley]

What
When getting the differences between two schemas, it might be also interesting to know if schema elements has been removed between two version. This may happen during optimization processes. However, detecting removed schema elements only make sense, if two versions of the same schema gets compared.

A remove difference would be identical to the add differences while it's body can carry the removed schema element.

Example

{
  changeType: "remove",
  schemaType: "Enumeration",
  itemName: "SimpleEnumeration",
  difference: {
    type: "int",
    isStrict: true,
    enumerators: [
    {
      name: "EnumeratorOne",
      label: "Enumerator One",
      value: 200,
    }
    ...
    ],
  },
}