Cocoa integration - fix outdated path in QCocoaFileDialogHelper

When we set accessory view sometimes (sic!) a delegate's callback
fires: -panel:directoryDidChange: with an outdated path (probably because
panels are shared?) resetting our current directory; later we open
file dialog with a wrong path as result.

Change-Id: Iffb02e801c44c5d9a62c2cca3acdf9278eaadb26
Task-number: QTBUG-50140
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@theqtcompany.com>