Move the code from QtWidgets/QFileIconEngine into a new class with
virtuals in QtPlatformSupport so that platforms can reuse it.
Prototypically use the class in the Windows and macOS QPA plugins.
Remove QPlatformTheme::fileIconPixmap() and change the type
of the hint QPlatformTheme::IconPixmapSizes from QList<int>
to QList<QSize> so that it fits better with the icon code.
Change-Id: I580e936f3507218757565ca099272cd575b3a779
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
2cd2cba086