Export (and possibly import as well) the entire archive as a .zip file

[herzbube]

To make this work cleanly, games should no longer be stored directly in the Documents folder, instead they should be stored in a subfolder.

Importing a .zip file is more difficult than exporting because the .zip file might contain just about anything. Importing should still be supported because it allows the user to use the export function as a backup, and the import function as a restore.

Some ideas for managing an import:

• Extract the content of the .zip file, then recursively iterate the resulting folder structure and import only files with the extension .sgf.
• If the archive has support for folders (cf. Add support for folders to the Archive view #63), preserve the folder structure in the .zip file
• Allow the user to specify whether she wants to replace the entire archive with the content of the .zip file