cmake-gui: Port away from deprecated QDirModel

QDirModel has been deprecated since Qt 5.15. !4894 (merged) switched from QDirModel to 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.

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