Use QEvent::ApplicationPaletteChanged instead of QEvent::PaletteChanged

Fix backported from Klassy.

When porting to Qt6 in commit 7e4ef7d7 ( !314 (merged) ) an event filter was added for qApp for event QEvent::PaletteChanged. However, Qt6 does not actually emit this event for qApp. The correct event isQEvent::ApplicationPaletteChanged.

See https://doc.qt.io/qt-6/qguiapplication-obsolete.html#paletteChanged See also a fix added in: frameworks/qqc2-desktop-style!316 (merged)