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.
The binding seems to handle that case. When I checked the "offline mode" and unchecked the "gadget" service, the IP address disappeared like it should. I tried a few permutations of this, and it seems to handle them all well.