Previously, the SelectListStore directly modified the GUI in its
update_items method, which was really just poor form.

This has been corrected by creating a new SelectList to encapsulate all
the GUI elements from the select-stack downward.  This handles showing
the select-none or select-frame elements as appropriate, as well as
adding the list item separators.  It's much nicer than the old way, if I
do say so myself.

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.