May make scrolling more laggy or jumpy.

This requires Qt 5.9 (to calculate overshoot).
The default scroll behaviour of list views has far too much inertia.
This should make scrolling feel more like scrolling the other scroll
areas of nheko.

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...