exported-object: add support for DBus ObjectManager interface

NMExportedObject now derives from GDBusObjectSkeleton, which is what
GDBusObjectManagerServer wants.  The main GDBusConnection and each
private server connection now gets a new GDBusObjectManagerServer,
and exported objects are registered with that instead of individually
exporting each GDBusInterfaceSkeleton.

Previously exported objects were not referenced by the BusManager,
but instead removed from the exports hash via weak references.  The
GDBusObjectManagerServer instead references exported objects, which
can make them live much longer than they did before.

Co-Authored-By: Thomas Haller <thaller@redhat.com>

src/dhcp-manager/nm-dhcp-listener.c

@@ -148,6 +148,7 @@ out:
 static void
 new_connection_cb (NMBusManager *mgr,
                    GDBusConnection *connection,
+                   GDBusObjectManager *manager,
                    NMDhcpListener *self)
 {
 	NMDhcpListenerPrivate *priv = NM_DHCP_LISTENER_GET_PRIVATE (self);