Use DBus to dispatch change notifications about the database from slaves (and from other running instances).

The Changeset objects are serialized as DBus custom types and emitted with identifiers of the sending process and the affected database.

Slaves emit signals with their object implementing the org.digikam.DatabaseChangesetRelay interface.

"Masters" emit signals with a different object implementing the interface, meant for peer masters, and receive signals from both.

Changesets are received and dispatched like local signals.