[User-Story] - Import multiple domain stories

[franksn90]

Story

As a dqedit user who defines mappings, I want to import multiple domain stories at once to add them to a single mapping.

Acceptance Criteria

• One to arbitrary number of DST files can be used in import
• Domain elements with same identity (type + icon + name) are merged into one element in the mapping
• All domain elements of all domain stories are part of the mapping

Dependencies

Additional context

This issue just requires to handle multiple domain stories at import, not adding single domain stories later (this is another issue).