Use QDBusServiceWatcher in QDBusAbstractInterface to track the owner

Instead of connecting to the "NameOwnerChanged" signal. This has better
performance, is simpler to implement and is future-proof: when we switch
to kdbus, there won't be a "NameOwnerChanged" signal to connect to. On
the drawback side, this will use slightly more memory.

Change-Id: I5801b8027949f041309567a493000fe7de9bf227
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>
Reviewed-by: Albert Astals Cid <aacid@kde.org>