nmtui mishandles terminal resizes

Current and all past versions. When you resize the terminal window, nmtui gets "randomly" broken: e.g. the blue background turns mostly white until the user makes it redraw by pressing a key. By playing around a bit more, you can achieve e.g. this:

Newt itself handles this better, as I've tried by running its demos--at least the background stays.

It'd be best if the code would recenter all windows using the current terminal dimensions. I'm not sure if it's malleable enough to do that, so I'll leave that as a bonus fix.