Replace unsupported characters in filenames when downloading

This is sort of a followup to http://www.soulseekqt.net/news/node/1261 - files with slashes "/" in their names can't be downloaded With regular Windows APIs you can't create or access files with \ : * ? " < > in them. If Soulseek tries to download a file with any of these disallowed characters in the name, the characters should be replaced on the receiving end. So please consider translating such characters to "_" so the files can still be created and used. Hope your new job is going well!