Draft: feat: display markers for MUCs

Uses the "read marker row" for MUCs.

Implements: #11275

Very basic GUI, adds a check mark in the message "header" (besides the "receipt" check mark, if enabled and present). Hovering on this check mark shows the list of nicknames of those who "read up to this point".

To do:

• Make it persistent across chat tab switching
• Prevent row merging depending on this check mark state
• Make it work with message correction: I think it works but not when you switch tabs (to be checked?)
• Fix tooltip

(the screenshot does not show the mouse cursor, but it's on the eye icon, looks like a 'text selection' cursor and the tooltip just does not appear)