Starts the Loader running. This will reset the progress and totals and then emit a start event.
If there is nothing in the queue the Loader will immediately complete, otherwise it will start
loading the first batch of files.
The Loader is started automatically if the queue is populated within your Scenes preload method.
However, outside of this, you need to call this method to start it.
If the Loader is already running this method will simply return.