Move the declaration of QFileIconProviderPrivate to it's own file.

Needed to backport qtbase/46685f755b01288fd53c4483cb97a22c426a57f0
to Qt4.

We can't add new symbols in Qt4 to QFileIconProvider so we will make
the private class a friend of the QFileDialog.

Change-Id: Ic720d681da14698ecf8557e1223d82bda6b33e23
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>
(cherry picked from qtbase/f4a0d6d2494d1dd41cd5b854a48b435120714d32)