Add IP address to status bar

This addresses issue ev3dev/ev3dev#184.

@dlech Here's my attempt at adding the IP address. I tested it and the IP label successfully unbinds and rebinds when the IP address changes.

Unfortunately, I wasn't able to emulate the feature in the desktop test, so it just shows "???.???.???.???" so that you can see where it would be. If you would like me to try to get it to work on the desktop, I can give it another shot.
Do we need to set  network_status_bar_item.ipv4_address to null so that it is cleared in case all services have been removed.