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

cclientdlg の closeEvent と OnAboutToQuit ハンドラーから始め、次に pull request #3372 の終了メソッドに関する議論を確認してください。そこで言及されているシャットダウン経路とハンドラーを追跡してください。終了動作が一貫して整理され、既存のシャットダウンの責務が引き続き正しい状態になれば完了です。