Expand demo_group_size.py

Include an example (or at least comment) when one half should *not* expand (replace HSplit by HGroup, and make the two sides springy = False and True, respectively). This would be particularly helpful when the fixed (non-springy) side is a Table, which "wants" to keep widening if given the chance.

https://github.com/enthought/traitsui/blob/master/examples/demo/Useful/demo_group_size.py