Support “Export” settings

Currently when you’re trying to export items, TMM keeps asking the user to select (the same) target folder and then overwrites existing files with the same filename in there without asking. If the file is being opened with a browser, then it fails to write the file. But there’s no message about that. You have to figure out yourself what’s going on by checking error log.

1. Support permanent export target folder setting.

2. Support editing of filename when exporting so that it could be customized.

3. If there’s a file with the same name or if the file is being used, please ask the user what to do next.