Support passing a QQuickPalette from C++ to Qml

In Qt5 a QPalette automatically converts to a QQuickPalette. In Qt6 QQuickPalette got much improved with support for different color groups. But now there is no way to initialize such a palette from C++ and pass it to Qml.

It is pretty useful to be able to generate your own palettes in C++, read them from a file or just have a property, that the Qml side subscribes to. One way to pass the palettes to qml, would be to automatically convert a QPalette to a QQuickPalette with the color definitions taken by looking at each ColorGroup for the different brushes. Otherwise it would be nice to expose QQuickPalette in C++ and maybe make it constructable from 3 or 4 QPalettes.