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Alembic import: option to always add a cache reader

The current behavior of the Alembic importer is to only create a
`MeshSequenceCache` modifier or a `Transform Cache` constraint to imported
objects if they have some animated properties.

Since static objects do not have a cache reader, when reloading files those
objects are not updated. Currently, the only way to properly reload a file
because of this is to reimport it.

This adds an option to the importer to always add a cache reader, even if
there is no animated data, to ensure that all objects coming from Alembic
archive are linked to them and updated properly upon reloads.

Reviewed by: brecht, sybren

Ref D10197.