QGuiApplication::paletteChanged(const QPalette &palette) [signal]

This allows QQuickSystemPalette to listen to palette changes without
installing an expensive event filter on the application object.

Change-Id: I8b693e047d993c444e393d7a714a5709692c3560
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>