[TASK] Provide information about import action in TCEmain to hooks

While importing T3D files the TCEmain currently fires the required
processing on records just as if the editor is generating the records. If
extensions hook into TCEmain, there is no way to differenciate this
situation from the regular editing mode - which might provide unexpected
results, i.e. wrong IRRE relations.

Solution is to provide a boolean to indicate that TCEmain has been called
during an import action which extensions could use to recognize this
situation.

Change-Id: I4fe429fb88f8ec24a7fda885f38ddfbdb4a642f5
Resolves: #36031
Releases: 4.5, 4.7, 6.0, 6.1, 6.2
Reviewed-on: https://review.typo3.org/10515
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters