Large Rework on Codebase

[s3inlc]

This PR brings various changes (mostly following the fact to better include the PHPUnit testing):

• It allows to now run PHPUnit tests to internally test the backend (outside of the exposed API).
• All classes now use namespacing according to the folder structure in the source code. This allows to get rid of most of the includes/requires (later all of them probably will be able to be removed).
• Cleaned up the mess in use statements (especially use statements in middle or lower parts in code files and not on top)
• API classes are now registered in the API index instead of the class files which gives more control when actually they are registered and allows better code inspection.
• Applied some further typing.
• All classes now live in their own file.

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