Enable option to see filename extension in Mac save dialog

Many Mac OS users expect to have the option to see the file extension
of the file being saved in the save dialog. This patch enables that
option.

Change-Id: I7713bcef16b6f43135b382c7107f306009c7a0a1
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>

src/plugins/platforms/cocoa/qcocoafiledialoghelper.mm

@@ -138,6 +138,7 @@ typedef QSharedPointer<QFileDialogOptions> SharedPointerFileDialogOptions;
         mSavePanel = mOpenPanel;
     } else {
         mSavePanel = [NSSavePanel savePanel];
+        [mSavePanel setCanSelectHiddenExtension:YES];
         mOpenPanel = 0;
     }