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

Start with cclientdlg's closeEvent and OnAboutToQuit handlers, then review the closing-method discussion in pull request #3372. Trace the shutdown paths and handlers mentioned there; done means the closing behavior is consistently organized and the existing shutdown responsibilities remain correct.

Written by the indexing model from the issue text.