Converted SpriteItem to QQuickItem

This is much more efficient than implementing a QQuickPaintedItem.

A single QSGTexture instance is shared for each ImageResource.

The textures are lazily created in the rendering thread, and deleted
using deleteLater. This should make sure they are not deleted while the
rendering thread may still be using them.

Reviewed-by: Erik Schilling
Renamed pixmapresource.{h,cpp} to imageresource.{h,cpp}

The class was already renamed in the previous commit.

Also cleaned up some includes in the process.