Add support for folders to the Archive view

[herzbube]

This feature would probably be worth a lot to many people because it would allow to organize an entire library of .sgf files. Things that should be taken into account:

• It's a lot of work!
• Competitors might hate Little Go for this
• Use a dedicated root folder that does not contain any other stuff (e.g. Documents/Archive). The current root folder is the application's Documents folder that may contain other stuff (e.g. the document interaction Inbox folder)
• Allow exporting a folder, or the entire archive as a .zip file via document interaction. This can be used to backup the archive.
• Importing should also be considered, however this must be designed carefully as a generic .zip file may contain all sorts of things. Design ideas:
  • Use .littlegoarchive as an extension instead of a .zip file. Of course this only serves to obfuscate and is no protection against manual intervention.
  • Scan the content of the .zip file, i.e. only import folders and files with extension .sgf
• Instead of, or in addition to, using folders a tagging system would also be useful