Load models asynchronously

[VictorKoenders]

When models are loaded they block the main thread. This can take up several seconds, especially in debug mode.

It would be nice if models were loaded asynchronously or in a background thread.

ModelHandle could have an pub fn is_loaded() -> bool { self.data.read().model.is_loaded } which allows the developer to check if the model is done loading. Iterating over the models that are not loaded would allow developers to show a progress bar.