libnm: make the the use of GInitable mandatory

Remove _nm_object_ensure_inited(), etc; objects that implement
GInitable are now mandatory-to-init().

Remove constructor() implementations that sometimes return NULL; do
all the relevant checking in init() instead.

Make nm_client_new() and nm_remote_settings_new() take a GCancellable
and a GError**.

examples/python/gi/get_ips.py

@@ -124,7 +124,7 @@ if __name__ == "__main__":
         sys.exit('Usage: %s <interface>' % sys.argv[0])
     dev_iface = sys.argv[1]
 
-    c = NM.Client.new()
+    c = NM.Client.new(None)
     dev = c.get_device_by_iface(dev_iface)
     if dev is None:
         sys.exit('Device \'%s\' not found' % dev_iface)