Fix setting top slot dimensions

We were drawing dimensions for the top slot from resize events, but
these ended up forcing a scrolling top slot to the size of the
containing window, not its contents, which was wrong.

Responsibility for this is now moved to the `ShoesLayout` itself, which
does a better job of it.

Fixes #617.
Get samples/expert-funnies.rb running again

With the background issue resolved, there's no reason for this sample to
be exclued.

Except it doesn't work :( XKCD appears to have discontinued the RSS
element that we were drawing our image from, and the code didn't handle
it gracefully.

So now we look to see if we get that parsed out right before adding a
section, so things are a bit more resilient.
Merge pull request #986 from shoes/top-slot-sizing-fix

Set the dimensions of a scrolling top slot correctly

* Fixes #617