Bump copyright year

NetworkHandler: make status callback private, reorder functions ...

The status callback function now gets privately declared at the earliest
possible moment, to both allow for reordering of code and avoid calling
this function directly from any other part of the code

* void changed to static void in status_callback declaration
* this now makes it possible to define network_init before
status_callback, as status_callback is declared much earlier

NetworkHandler: All access to the underlying NetworkInterface ...

We need this to actually use the network.

NetworkHandler: replace redundant include of EthernetInterface ...

The NetworkHandler header already includes EthernetInterface.h, so this
was redundant. It's also better in the future to not have the
NetworkHandler code directly require EthernetInterface.h, to allow for
versatile usage.

NetworkHandler: added a new function to check weather we're connected ...

This now uses an atomic boolean to be thread safe.

Added networking support ...

Networking is handled by the NetworkHandler portion of the code, which
provides a few functions for use by other parts of the program, and a
status callback to monitor for connectivity changes. Connectivity is
also handled in an asynchronous mode, to avoid freezing the entire OS
while actions are performed.

* network_connect should be called by any part of the program wishing to
establish connectivity
* network_disconnect should be called to disconnect the network once
done
* network_init is a special function which should only be called by
main. It is responsible for setting the network interface in
asynchronous mode as well as attaching the status monitor callback. No
other part of the program should ever call this.

Lwip is used for the network stack, and IPv6 support has been enabled,
and prefered over IPv4 if available.