Implement file-based import to enable clonability of import-data to speed up multi-target imports

The goal of this Issue is to reduce the strain and speed up the import of multi-target tasks. Currently, each resulting task is imported seperately. What needs to be achieved:

• The Import is changed so it uses one (or several) JSON file(s) as data-container for the task throughout the import (which would radically reduce DB-strain -> Deadlocks).
• In a later/final step, this file is flushed to the DB-Model and t5 acts "as we know it"
• This would enable a "natural" capability for cloning tasks based on these file(s) and triggering a pretranslation afterwards for the sibling tasks after the first child-task is imported.
• Throughout the file-based process, the segmentNrInTask acts as segment-id.
• A potential Pivot-workfile also has to be processed to a file-based (separate) model to be re-usable for the sibling-tasks
• Also some Plugins (Visual, ConnectWorldserver ) need to be adjusted to implement file-based import & clone-APIs

This Feature obviously needs a careful planning and a separate estimation, the above is just a rough idea. It can be implemented by Thomas, Axel, Aleks, Sasha and may is a great candidate for team-development.