WIP: New ConversationView

This is a Gtk.Listbox implementation for Gajim’s message display. Some parts are taken from !678 (closed) (credits to @eta), and some ideas originate from @ehuelsmann ’s Wiki page. Since this only modifies displaying mechanics, it should be safe to test (and to switch back afterwards). Gajim’s history view still uses the old ConversationTextview, enabling testers to compare how messages are displayed.

I added TODOs in the code comments at places where either additional work is necessary or where a joint decision has to be taken. Both message timestamp and nickname are formatted via settings (in ACE: before_nickname, after_nickname, and time_stamp). For time_stamp, %H:%M will render similar to what’s shown in the screenshot below.

• Fixes #9683 (closed) (dynamic history loading)
• Fixes some points mentioned in #9060 (closed) (e.g. avatars, text indentation)
• Adds a ReadMarkerRow showing how far your contact read your messages

Note that plugins (e.g. URL Image Preview, Syntax Highlighter) need to be adapted to this approach, and don’t work yet.

Example screenshot: