This requires a QAbstractFileIconProvider in QtGui, as the standard
QFileIconProvider depends on QStyle, and cannot be moved out of
QtWidgets.
QAbstractFileIconProvider returns strings for file types, but returns
no icons yet. Support for a default icon set might be added in a
follow-up commit.
Change-Id: Ib9d095cd612fdcf04db62f2e40709fcffe3dc2b7
Fixes: QTBUG-66177
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
