Ticket #379 reports crashes on quit due to `willSavePrefs` being called
when result and details dialogs are already freed. I can't reproduce the
crash, but it's still a bad idea to rely on the timing of
`aboutToQuit()` to launch this process.
This commits uses a more predictable place to emit `willSavePrefs` and
I'm pretty sure it will fix the crash at #379.
