cmake-gui: Port away from deprecated QDirModel
QDirModel has been deprecated since Qt 5.15. !4894 (merged) switched from
QFileSystemModel, but !5619 (merged) reverted back to
QDirModel due to #21589 (closed). See #21589 (comment 873570) in particular.
!5619 (merged) left a FIXME comment to resolve this while not breaking completion of partial path edits.