Removed unused files from export extension.
Floaters use API to retrieve data.
Pretty print all REST API responses.
Added Swagger documentation for REST API.

Also fixed some minor bugs in the API.
Many API improvements.

- removed base api to allow swaggerPHP to set it correctly
- fields filter works on nested elements
- aligned swagger documentation of api with implementation
- allow sorting in all lists
- check for required fields when creating or updating entities