The attached demodalize.patch (which includes the changes from the previously posted decouple_config.patch) changes both the settings dialog and the about dialog to be non-modal.

There's a new class UIFactory which is responsible for launching these kinds of windows, keeping track of them, and making sure there's only one of each kind at a time. This is also where the logic for choosing ttk vs. non-ttk components will go.

As a (questionable) bonus, the about dialog, which now incorporates the README's etc directly into the window rather than launching further modal dialogs, also has some other minor cosmetic changes.