Avoid transforming data after every file when loading multiple source data files

[JohannaOtt]

When using the reader mechanism to load multiple files at once and the live transformation is enabled, the loaded data is transfomed after every file imported.
This should be changes so that the data is only transformed once all files are loaded because the "in-between-transformations" are not of interest for users.

[github-actions]

This issue has been automatically marked as stale because it has not had activity in the last 60 days. It will be closed in two weeks if no further activity occurs. Thank you for your contributions.

[thorsten-reitz]

This should not be closed automatically.