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Fix T59164: Run Alembic in background when INVOKE'd

This is similar to what physics baking is doing: invoking the operator
runs a background job, whereas executing blocks. This makes Python
scripts calling the Alembic import/export operators more predictable.

For backwardward compatibility with existing Python code the
`as_background_job` parameter still exists, which overrides the
behaviour chosen by INVOKE/EXECUTE.

Reviewers: brecht

Reviewed by: brecht

Differential revision: https://developer.blender.org/D4137/new/