This method will center a group of window rects, within some
bounds, accounting for the properties of the currently selected theme
(i.e. border width, title height, etc).
There are two different things in ThemeEditor that want to know when a
palette changes:
1. The PreviewWidget subclass, so it can update its preview.
2. The ThemeEditor itself, so we know that the palette is modified.
Using a protected virtual function for 1 means that we can do 2 without
them fighting over the same on_palette_change callback.
