Allow opt-out of adding application name to window title

On at least X11, Wayland, and Windows Qt automatically appends the application name to the window title.

Consider this code

QApplication::setApplicationDisplayName("My App");
QApplication a(argc, argv);
MainWindow w;
w.setWindowTitle("My Window Title"); 

which on Wayland gives the window title "My Window Title — My App".

This behavior is wanted and fine most of the time.

However, there are cases where the application developer wants tighter control over the window title and omit the application name.

One such case is KDE Plasma, where we use Qt to implement several system-level dialogs. The name of these "applications" is an implementation detail that does not concern the user and thus should not be shown. We need a way to make setWindowTitle() set the title verbatim without appending the application name.

See the downstream reports https://bugs.kde.org/show_bug.cgi?id=466059 and https://bugs.kde.org/show_bug.cgi?id=466056