DesktopStartup holds the WinForms main window's start-up steps in its
order for the Avalonia app: first the one-time conversion of the legacy
nest template into the nest defaults file, then whether Tools > Options >
Create a new nest on startup asks for a new nest.
Run reads the shared user settings but never writes them, so it can run
off the UI thread; it reports whether to clear NestTemplatePath, and
ClearTemplateSetting empties that setting alone on the caller's (UI)
thread, as the window's other settings writers run there.
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