Previously, the device list only loaded once on startup.  This was poor
behaviour, as devices can be attached and detached at any time.

Now the attached devices are polled once per second, and the list is
updated accordingly.  The code behind it is a bit messy because the
list model interacts directly with parts of the GUI, but it works
pretty well nevertheless.  There's no mechanism to kick us out of the
configuration screen if the device we're configuring is detached, but
this can be addressed later.  For now, I'm reasonably happy with this
new feature.

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.