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.
One is the actual IP address, which will always be parse-able as an IP address (although, now that I think about it, I'm not sure why one would want to parse it). The `text` variable is what's actually being displayed, so we could later add logic that would add a message such as "not connected" if there was no address to be displayed. Should I change it to use just one instead?