Add Cx helper to spawn observer

This is what I arrived at trying to move to the bevy_picking way of doing interactions. It ends up moving the observer setup out of the Element builder, for better or worse. Usage looks something like this ``` let id = cx.create_entity(); cx.create_observer(move |_click: Trigger<Pointer<Click>>| { ... }, id, ()) Element::<Button>::for_entity(id) ```