Allows to import multiple alembic files in single operator call.
When importing multiple alembic files as background job, import progress
will be divided by all files, so if `5` files are imported, each file
will make progress of `20%`. This can be improved if the job text can be
customized to display for example `Import Alembic 1/5` and using 100%
progress status display for each file, but that is out of the scope of
this pr.
The Scene min and max frame are set based on the minimum/maximum frame
ranges detected from all files.
Pull Request: https://projects.blender.org/blender/blender/pulls/121492
5e9d19d58b