Distribute Qt with ICU

Distribute (at least for the desktop platforms - Windows and Mac) Qt with ICU enabled/included. As for now, Qt with ICU is only available on Linux and the "official" Windows (tested) and Mac (AFAIK) builds are shipped without ICU.

This means that several important things like XML Stream reader (see QTBUG-86437 for details) do not work properly on Windows in Qt6 when the official installers are used. To force people to build their own version of Qt just to get a very basic functionality that works fine in Qt5 can IMHO not be justified by the increased Qt distribution size, at least not for the desktop platforms where the size is not crucial today. The difference between the platforms where on Linux opening XML files works fine but fails on Windows is also not very nice...