Refactoring/switch to filesystem abstraction

[cs-util]

Now all methods related to file handling use a file system abstraction that allows to work with different scenarios for file handling, e.g.:

• Zip files can now be handles the same way as normal folders (for now only the ready only operations will work)
• A .net backend can now implement its own filesystem to be able to use all exsting filesystem related methods. It could for example use an in memory file system or a remote ftp system
• file pathes will now by default be relative so that for example if such pathes are persisted json moving the persisted json plus the files to a new parent folder will not break anything

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This change is 

[codeclimate[bot]]

Code Climate has analyzed commit d3271e86 and detected 7 issues on this pull request.

Here's the issue category breakdown:

Category	Count
Complexity	5
Duplication	2

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