#36 PluginGUIs layering and placement

We should have some measure of "intelligence", in the
code that displays the Plugin GUIs.

At the moment new plugins appear in a set place on the
screen, and this can be under another plugin.

When GUIs get layered on top of each other, sometimes
widgets from the lower GUI "shows through" and
interferes with the higher GUI.

Both of these behaviors are a bit confusing and ugly,
and we should do something to improve this situation.