REG->6.0/Windows : QFileSystemModel no longer displays file icons

Steps to reproduce:

• Launch tests\manual\dialogs
• Turn off native file dialogs
• Open File dialog
  Note: No icons are displayed

Note: The dirview example is not suited for reproducing this since QAbstractFileIconProvider as a result of moving QFileSystemModel to QtGui no longer creates any icons on Windows. https://codereview.qt-project.org/c/qt/qtbase/+/334113 attempts to fix this. One needs to instantiate a Widgets' QFileIconProvider and set it on the model (see qtbug91104_dirview_repro.diff , which can also used to show pixmaps via context menu ).

Analysis so far ( qtbug91104_log.txt ) shows that QWindowsFileIconEngine (src\plugins\platforms\windows\qwindowstheme.cpp:885 ) apparently no longer works after the recent QIcon changes. It still serves the correct pixmaps, but QPixmapIconEngine::scaledPixmap() returns a null pixmap ( src\gui\image\qicon.cpp:298 ).