Reorg dirs: remove cyclic dependecy of `GUI/Models` on `GUI/DataLoaders`

t.knopffrequested to merge
reorg_dirs into develop

Currently GUI/Models is dependent on GUI/DataLoaders and vice versa. In this MR the dependency of GUI/Models has been resolved in the following way:

  • the abstract interfaces of the data loaders AbstractDataLoader, AbstractDataLoader1D, AbstractDataLoaderResultModel and the data loader registry DataLoaders1D have been moved to GUI/Models
  • Registration of the data loaders has been rewritten to make the registry independent of the concrete data loaders.

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