core: refactor NMBusManager to hold reference to NMExportedObject directly
Previously, nm_bus_manager_register_object() would take various D-Bus skeleton objects that were associated with one NMExportedObject. This was confusing, in that these skeleton objects are all for the same NMObject but correspond to different D-Bus interfaces. Also, setting the D-Bus property "Managed" of a Device is broken because we might retrieve the wrong skeleton. Now, the NMBusManager has a reference to the exported object directly. The skeleton interface instances instead are now exposed by the NMExportedObject.
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