Fix desktop implementation of UserSettings Module

[ichan-mb]

Currently broken on desktop (NET) because:

• There is no Json.Stringify for Dictionary object (the implementation is using Dictionary object and serialize it to json)
• Shared output file between Apps / Project, so it will overwrite the output between Apps/Project

This will fix those problems.

This PR contains:

• Changelog
• Documentation
• Tests

[mortend]

The build is failing on AppVeyor, but it looks like a glitch/false negative.

Can you try force-pushing the branch to trigger a new build?

git commit --amend
git push --force

[ichan-mb]

seems OK now..