Remove per-collection sync progress, unused and slow.

It was only used by the QML GUI, and it triggers a large amount of dataChanged
signals in a model that has many many layers of proxymodels on top.
ChangeRecorder: Fixed dumpNotificationListToString

Instead of reimplementing the change file reading (including bugs), reuse what is there already.
ItemModifyJob: support an empty set of tags.

Required to clear tags.