Thanks to red_sky for the feedback!

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...

I suck at sizing so the images in the message are currently hardcoded to
300 pixels in width...

Text selection over multiple items doesn't work yet