Ensure the filedialog shows the initial filename

Whenever the selection changes it will call panelSelectionDidChange
even if the panel is not visible which causes it to overwrite our
chosen initial file with "untitled". Therefore we only set the name
when the panel is actually visible.

Change-Id: I09f7ebabce5c2139a7f74b953391d39dfad65db2
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

src/plugins/platforms/cocoa/qcocoafiledialoghelper.mm

@@ -446,7 +446,7 @@ static QString strippedText(QString s)
 - (void)panelSelectionDidChange:(id)sender
 {
     Q_UNUSED(sender);
-    if (mHelper) {
+    if (mHelper && [mSavePanel isVisible]) {
         QString selection = QCFString::toQString([[mSavePanel URL] path]);
         if (selection != mCurrentSelection) {
             *mCurrentSelection = selection;