Rewrite cclientdlg closeEvent, onAboutToQuit, ...

There was a lot of discussion on closing methods in cclientdlg in https://github.com/jamulussoftware/jamulus/pull/3372 This should probably be refactored.

> Maybe `closeEvent` should just call `QCoreApplication::instance()->exit();` and then do the work in `OnAboutToQuit` handlers? That should stop needing to call `XYZ.Stop()`.

> Of course, if that's what the "default implementation of this event handler routine" does, it would move all the code here into `ClientDlg.OnAboutToQuit` (plus handlers in each called class).

_Originally posted by @pljones in https://github.com/jamulussoftware/jamulus/pull/3372#discussion_r1763615127_, https://github.com/jamulussoftware/jamulus/pull/3372#discussion_r1763580248, https://github.com/jamulussoftware/jamulus/pull/3372#discussion_r1759707896

Inizia dagli handler closeEvent e OnAboutToQuit di cclientdlg, poi esamina la discussione sui metodi di chiusura nella pull request #3372. Traccia i percorsi di arresto e gli handler menzionati; il lavoro è completato quando il comportamento di chiusura è organizzato in modo coerente e le responsabilità di arresto esistenti rimangono corrette.

Scritto dal modello di indicizzazione a partire dal testo della issue.