D-Bus adaptors should not export property notification signals

When exporting an adaptor with the QDBusConnection::ExportAdaptors flag, all of its methods, properties and signals are exported – it's impossible to restrict the set of exported signals by using, for example, QDBusConnection::ExportScriptableSignals.

This can be a problem for several reasons:

• If the adaptor is deriving from some other class, all of the parent class' properties, methods and signals are also exported (except for those defined in QObject, which is hardcoded to be excluded)
• Property notification signals are most likely not meant to be exported: D-Bus has its own PropertiesChanged signal, and property notification signals might have been added just as an internal tool to watch for changes and emit the PropertiesChanges signal

All the above is solvable by not using QDBusAbstractAdapter, but then it becomes impossible to handle more than one interface at the same object path (which is something I need).