qt: disable using top level (independent) window for tool tip until Qt 6.8.2 with non-contrib Qt

This feature has not been tested properly by Qt, it often causes crashes.

We first started using this feature with !6240 (merged). First, I came up to #28901 (closed) (https://bugreports.qt.io/browse/QTBUG-131898). I thought that the crash would be limited to that one, but now there is another #28919 (closed)...

I propose not using this until Qt 6.8.2. At least the former issue has already been fixed upstream (to be included in Qt 6.8.2, and is already in our contrib Qt with !6494 (merged)), and if @JohannesKauffmann reports, I believe that they will fix #28919 (closed) as well before Qt 6.8.2 is released (fix for that for contrib Qt should be !6597 (merged) as an interim solution).