highlight tabs only on unviewed messages

[hemirt]

This draft tackles the problem of unnecessary higlighting of images viewed in currently selected notebook tab, as discussed here: https://github.com/Chatterino/chatterino2/discussions/5412

It only tackles the part 1) do not highlight other tabs on messages shown in current tab, does not yet tackle the part 2) on selection unhiglight possible other tabs.

This is just a proof of concept, just to see what might be necessary to do these changes and to tackle the ideas.