show physical page ids instead of thumbnail image file ids

[bertsky]

In the navigation view on the left where the thumbnail images are displayed an pages can be selected, pages can only be identified via their file ID (and the contiguous integer in between the navigation buttons).

However, I would find it more useful to display the physical page ID there. (Image file IDs can still be shown as text overlay in the image view.)

This became more pressing with METS files from Kitodo.Production v3, which uses UUIDs instead of contiguous numbers for pages and files, e.g. http://digital.slub-dresden.de/id1883722624-19520000 (to browse this, you first need to create an OCR-D workspace from the OAI link via ocrd workspace clone, then download ORIGINAL and FULLTEXT files, convert the latter to PAGE via ocrd-fileformat-transform -P from-to "alto page", finally use this script to fix the empty @imageFilenames).