Use default depth window icon when application runs on Exceed X server

If a Qt application is running on the Exceed X server, Qt does not detect this, and treats this as an unknown environment. Window icons on unknown environments are converted to 1-bit images.

However, Exceed is capable of displaying higher depth window icons, so it would be great if Qt could detect this and use a default depth window icon when running on Exceed.

This used to work in Qt 3, where window icons were not converted to 1-bit.