Use QEvent::ApplicationPaletteChanged instead of QEvent::PaletteChanged

When porting to Qt6 in 7e4ef7d7 an event
filter was added for qApp for event QEvent::PaletteChanged. However, Qt6
does not actually emit this event for qApp. The correct event is
QEvent::ApplicationPaletteChanged.

See https://doc.qt.io/qt-6/qguiapplication-obsolete.html#paletteChanged


(cherry picked from commit 232ff8ac)