A way to add categories to all checked items

[prototyperspective]

Useful for similar reasons and a workaround to this: #163

petscan can be extremely useful to find media files for new categories on Wikimedia Commons or to find items still missing in some existing category and many other applications like that.

The problem is one has to manually open each file in a new tab and add then add the category at the bottom (or top depending on settings) of the File page using HotCat. In contrast, on Wikimedia Commons one can use the Cat-a-lot gadget to quickly add or remove categories from many files that for example were queried using the search engine with search operators.

One can already do batch-wise changes to checked items for Wikidata properties using this box:

Please add a way to do the same for WMC categories. This would be very useful, especially for maintenance. One would log in with OAuth and then execute the configured changes on the checked items.

When using the thumbnails view one could also use it to add categories by things visible from large thumbnails for individual files such as which year an "Our World in Data" map shows data of (the year is usually in the headline).