provide region level for line segmentation

[bertsky]

Since Detectron2 based models for textline segmentation have appeared, we should support them by adding a operation_level=region. Also, the cat2class mapping must be extended by TextLine type to allow its use in the categories parameter.

(However, ideally there should be some kind of consistency checking between categories and operation_level: tables can only be added on page level, cells only on tables, lines only on text regions...)