qt, contrib: get rid of qt5compat

Qt5Compat.GraphicalEffects is now only used if Qt version is less than 6.5.0. This makes qt5compat not relevant for contribs, as we provide Qt 6.8.0 currently (QtQuick.Effects is used instead).

I also fixed the csd window shadow that is stacked below the interface not being hollow. This is currently a problem when the window has backdrop blur so that the interface has translucent parts. CSD shadow should not act as a filter for the backdrop blur effect.

Custom rectangular glow patches picked from !5417 (closed).

Request review @chub.