More layout work for stats tab

This finally gets me something that I like. The way the different boxes
get resized seems pleasant and intuitive. And it appears to do the right
thing on all platforms (I learned that having different levels of layout
nested creates no vertical offset on Linux/KDE, but a very ugly offset on
Mac, for example).

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>