Support complete offline mode

[imLinguin]

Allow users to specify that they don't want to upload achievements, stats and leaderboards back to GOG.
Some people asked me this personally. I believe it's a nice addition to consider

It is not possible to support this fully offline as we need to load the data at least once

Check-list

• Add ability to disable achievements upload
• Add ability to disable stats upload
• Add ability to disable leaderboards upload
• Add ability to export the data for easy access and management in external tools
• Add ability to import previously exported data