Thanks to red_sky for the feedback!

This also makes long messages unreadable, because we don't shorten long
usernames anymore. We may eventually want to do that again, but it is
hard with translations and we probably want to shorten the displayname
more, as before this change the message was only ever as long as the
timestamp, which is usually just 5 characters...

The interface is taken from Qt/KDE, but the implementation is different,
because the Qt implementation depends on some Qt internals.

Pagination could get stuck, if the messages request failed.
Section height seemes to have been calculated to late, which would make
some section overlap the next message in some cases. Fix that by doing
the height calculation manually.

requires Qt5.12+

Size is fixed for now, otherwise the Video output ends up somewhere...