Call endSheet when the QFileDialog was shown as a sheet on Mac

If the sheet is not ended then subseqent calls to show a sheet will not
work correctly.

Change-Id: Ib8a43a1c96a3dadff196c433e822f7579ad87b8b
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>

src/plugins/platforms/cocoa/qcocoafiledialoghelper.mm

@@ -218,6 +218,8 @@ static QString strippedText(QString s)
     *mCurrentSelection = QT_PREPEND_NAMESPACE(QCFString::toQString)([[mSavePanel URL] path]);
     if ([mSavePanel respondsToSelector:@selector(closePanel:)])
         [mSavePanel close];
+    if ([mSavePanel isSheet])
+        [NSApp endSheet: mSavePanel];
 }
 
 - (void)showModelessPanel