Thread annotation

[charlie17]

Allow for "thread annotation" - for humans only - for example to leave a note for oneself about the thread, where you are leaving off with it, what to come back for. Add eg "Notes" as an option to the r-click menu:

[juliusmarminge]

Closing as a duplicate of #8410, which already tracks human-written thread notes, including editing them from the thread context menu.