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.
This is fine for now. Eventually we will want to add something to the control panel to be able to change this value. It is actually better if we don't do it like I described in my email, but rather just add an Entry where you can set this to an arbitrary value.

And on second thought, instead of question marks, let's set this to our default Windows/USB RNDIS IP address of 192.168.137.3.