More QUrl/QString conversion in FileDialog implementation

Followup to 6499098a04c7a8a1193413500cd562f781df38bb :
because the signals in QPlatformFileDialogHelper emit URLs and
QFileDialog does not provide compatible signals, the bridge class
QFileDialogHelper must provide slots to convert strings to URLs and
emit the signals.  Therefore QFileDialogHelper needs the Q_OBJECT
macro, and therefore it helps to move its declaration to the private
header to avoid confusing moc.

Change-Id: I088ff73d107a460af3f8695401276b45a66b5d59
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>