QFileDialog/win: pass only filename to IFileDialog::SetFileName()

When calling QFileDialog::selectFile(), the file is passed as QUrl to
the underlying functions. Then this url was converted back to a local
file and the full path (with wrong path separators on windows) was shown
in the filename edit box in the native dialog. There was already a
comment to use QUrl::fileName() but it did not exist when the code was
written and later it was forgotten.

Fixes: QTBUG-47159
Change-Id: I478dfd0e4943277c0ac78ebf4e138257e7ea8662
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
(cherry picked from commit 19c701d370512e81d1b934f899b03ae0c2e6953f)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 4938b234ad251c5b2480bb9e28b3482fb4425345)

src/plugins/platforms/windows/qwindowsdialoghelpers.cpp

@@ -1585,7 +1585,7 @@ void QWindowsFileDialogHelper::selectFile(const QUrl &fileName)
     qCDebug(lcQpaDialogs) << __FUNCTION__ << fileName.toString();
 
     if (hasNativeDialog()) // Might be invoked from the QFileDialog constructor.
-        nativeFileDialog()->selectFile(fileName.toLocalFile()); // ## should use QUrl::fileName() once it exists
+        nativeFileDialog()->selectFile(fileName.fileName());
 }
 
 QList<QUrl> QWindowsFileDialogHelper::selectedFiles() const