[Summary] On MacOS allow to create folder in new_session_dialog

[Reviewed] GZharun

[git-p4: depot-paths = "//Abdaw/dev_main/tracks/": change = 463589]

gtk2_ardour/open_file_dialog.mm

@@ -108,6 +108,7 @@ namespace ARDOUR
     // Create a File Open Dialog class.
     NSSavePanel* saveDlg = [NSSavePanel savePanel];
     [saveDlg setTitle:title];
+    [saveDlg setCanCreateDirectories:YES];
     
     NSFileManager *fm = [[NSFileManager alloc] init];
     BOOL isDir;