Refactor PAC-96 image override option

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PAC-96 introduced a new option which allows overriding already existing images. In order to stay backwards compabile, new methods and attributes were made private in
src/Subjects/FileUploadTrait.php. These should be either public or protected.

Additionally, a new constant OVERRIDE_IMAGES should be introduced in the ConfigurationKeys utility class for components using the new option:

• techdivision/import-attribute
  • src/Subjects/OptionSubject.php
  • src/Utils/ConfigurationKeys.php
• techdivision/import-category
  • src/Subjects/BunchSubject.php
  • src/Utils/ConfigurationKeys.php
• techdivision/import-product
  • src/Subjects/BunchSubject.php
  • src/Utils/ConfigurationKeys.php

The necessary changes are highligted as "TODOs" in the corresponding files.