Add a way to stop the process of Loading chunks

Currently, there is no way to stop Chunky from loading chunks after clicking `Load selected chunks` or `Reload chunks` outside of closing Chunky completely. A way to stop the loading would be useful, especially when you have a scene set up, and you load very many chunks, but then need to adjust a setting. It would be frustrating to either wait long for Chunky to finish loading, or close Chunky and then have to set everything up again.