Ability to cancel XKT model loading

[xeolabs]

When using XKTLoaderPlugin#load, make it possible to call PerformanceModel#destroy() on the model, while the model is still loading. This is effectively a "cancel loading" capability.

Previously, we needed to actually wait for a model to finish loading before we can call destroy() on it, as shown below. The problem here is that the whole model (and possibly metadata) needs to be loaded into the Viewer before we're allowed to destroy it, which is wasteful and might create a rough user experience.

const model = xktLoader.load(...);

model.on("loaded", ()=> {
    model.destroy();
});

Now we can just do this, if we need to:

const model = xktLoader.load(...);

model.destroy();