Windows: Introduce QFileDialog::DontUseCustomDirectoryIcons

Folders can have a custom icon, set by the user. Some system
folders also have one, for example c:\windows\fonts.

This option allows you to disable this behavior, you'll get the
folder directory icon.

As a side-effect, you'll get a very big performance improvement
on removable/network media: 2 seconds vs 60 seconds on a SDCard
with 10000 folders.

Unlike in Qt5, here we didn't add new methods to QFileIconProvider
do avoid introducing new symbols.

Change-Id: I0b98a180e53c9ee7d583f19b3917fcc9166cb3eb
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
(cherry picked from qtbase/46685f755b01288fd53c4483cb97a22c426a57f0)
(cherry picked from qtbase/149f3efc39bebbe8fceacf2553836f90d2e1c383)
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>