Rewrite cclientdlg closeEvent, onAboutToQuit, ...

[ann0see]

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