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/show-wifi-networks.py

@@ -62,7 +62,7 @@ def print_ap_info(ap):
     print
 
 if __name__ == "__main__":
-    nmc = NM.Client.new()
+    nmc = NM.Client.new(None)
     devs = nmc.get_devices()
 
     for dev in devs: