Refactor/change management in transaction

[tejaede]

This is the implementation of the solution described here: #114 (comment)

A couple of notes

• DataService.handleChange does 2 things.
  • 1. Pass the event to the transaction managing the object
  • 2. Set the inverse values
• DataService methods that manipulate createdDataObjects, deletedDataObjects, changedDataObjects, and dataObjectChanges are now just proxies to a transaction. For example...

        registerChangedDataObject: {
            value: function (dataObject) {
                let transaction = this._transactionForObject(dataObject);
                transaction.registerChangedDataObject(dataObject);
            },
        },