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.
Yes, just use one. I've written this using the Model/View/Controller pattern. The views (which this is a view) are pretty "dumb", so it should just display whatever we tell it to. Any logic should be at the controller level. Unless you want to add logic here that displays something like "not connected" if we set the value to null.