Support synchronous image loading and QPixmaps in QDeclarativeImageProvider

QDeclarativeImageProvider should be able to load images synchronously as well as asynchronously.

It should also be able to load pixmaps as well as images. Note that pixmaps can only be loaded synchronously as they must be created in the main thread. This feature will allow the Image::pixmap property to be removed, as pixmaps can be distributed via QDeclarativeImageProvider instead.