Showing the selection checkboxes also in the thumbnails view

[prototyperspective]

Currently, these are only showing in the titles view. Theoretically, one would have to switch back and forth and check which files are checked instead of un/checking directly in the thumbnails view.

This is related to #174 in that it would make that way more useful.

I described an example of how the thumbnails view could be used to add categories at the bottom of #173. Another example is that in this scan (simplified) one could add items to category "AI-generated media" from the thumbnails view (and then go there and subcategorize each further; this was just an example for illustration).