Added ResourcePointer class

Signed-off-by: Irmo van den Berge <bergerkiller@gmail.com>

This should make it easier to pass around resources without having to load them. It also allows a better way of storing resources as constants without loading them, but still allowing them to be loaded in a user-friendly way.

This should make methods like setModel easier to implement, because you can force a resource type. This is not possible with the String or URI types. (well, you can, but with an exception at runtime...)