WIP: Convert SpriteItem to QQuickItem

This is much more efficient than that implementing a QQuickPaintedItem.

TODO: Do the conversion from QImage to QSGTexture only once per image.
TODO: Make sure the QSGTexture gets deleted properly.
TODO: Switch QSGTexture when required by the animation frame.
Share a single QSGTexture instance for each ImageSet

Using QSharedPointer with a custom deleter, this should automatically
free the texture as soon as there are no scene graph nodes referencing
it anymore and the ImageSet is disposed, while still being deleted in
the right thread.