Port away from deprecated paletteChanged signal
See also: frameworks/qqc2-desktop-style!224 (merged)
Drop support for Qt older than 5.13
#if QT_VERSION < 0x050D00 // Check if Qt version < 5.13 else if (object == qApp && event->type() == QEvent::ApplicationPaletteChange) { configurationChanged(); }
Minimum supported version specified in CMake is 5.15.2, so this conditional code is clearly not relevant anymore.