[wp-ui] Tabbed theme interface mockups

[Gordon Brander]

I believe something along the lines of the first comp to be a better solution.

Let's try to zoom out from this problem a little bit and think of ways to extend and standardize the WordPress Admin's design language. In future, tabs may be a good solution in another area of the admin too. Instead of solving this *specific* problem, let's solve the problem of what tabs should look like throughout the interface.

—Gordon