dbus: Q_DECLARE_METATYPE must be called with qRegisterMetaType<>

From the documentation:

    To use the type T in QVariant, using Q_DECLARE_METATYPE() is
    sufficient. To use the type T in queued signal and slot
    connections, qRegisterMetaType<T>() must be called before the
    first connection is established.

I don't really understand why it was working with Qt 5.15, but the
code was not working with Qt 6, so add qRegisterMetaType to fix
signals.
Moreover, previous qRegisterMetaType<> in ContactModel was
misplaced.

jami-client-qt#601

Change-Id: I309de4b980d961ac352529419f895be369a87433