Since merging,

   http://cgit.freedesktop.org/NetworkManager/NetworkManager/commit/?id=18ecf48d7a9d03194db1b65ef46e386284426f89

we support a scheme to set default values for connections.


Currently we inject the default values on the fly, whenever needed.
E.g. ip6-privacy:
  http://cgit.freedesktop.org/NetworkManager/NetworkManager/tree/src/devices/nm-device.c?id=5eba53cd53a25ebf104db2cfba1eddff0aa9edd6#n4780


That is a bit hacky... we can do better.


After we get Lubos branch lr/applied-connection-bgo724041 (bug 724041), there is a point in time when we clone our NMSettingsConnection to get an applied-connection.

At that moment, we should fill all unset values from the applied connection
with our default values.

That is better because:

  - once we fill/complete the applied connection, it stays unchanged.
  - we have one function that does the default lookup, instead of spreading
    calls to nm_config_data_get_connection_default() all over the place.
  - since the applied connection can be retrieved via D-Bus, you can
    actually lookup how the defaults were filled.
  - we can do more complex ~completing~. Currently we can only
    overwrite very simple properties. If we have one ~fill~ connection,
    we might even be able to fill complex properties such as "ipv4.method".


Also, for nm_connection_diff() we need a new diff-flag: "ignore-default"