It had been stretched vertically due to the two text labels stretching
the whole grid.  Now the button is square, because it's set to align
center rather than fill.

It's by no means wonderful internally, but to a large extent it works.
The list functionality is still untested, since I won't be able to have
two PDB Sinks connected to my computer at once until tomorrow.  But for
now, a neat little GUI.