Let Ember be able to unload unused resources

To free up memory Ember should be able to unload unused resources when needed. This would mainly apply to meshes and models which haven't been shown in a while.
A manager would need to keep track on the oldest unused resources which could be unloaded and at regular interval perform unloading. One has to be careful to not be too aggressive, to avoid swapping as the user moves through the world and unloaded resources are loaded again.