Update loading in a thread demo for 4.0

[jtnicholl]

This updates and improves the simple background loading demo (the one that doesn't use load_interactive). On the list in #697.
In addition to updating for 4.0, I made the following additional changes:

• Make the comments more clear and informative. I removed a few that I thought were just stating the obvious (like one after return tex that just says it returns something, or the one that says ResourceLoader.load loads a resource). Most of them I kept and just edited for clarity.
• Don't reuse Thread objects. According to the documentation, you shouldn't do this, so the demo now waits for the previous thread to finish (if there is one) and makes a new one.
• Add an _exit_tree method that makes sure the thread is finished, then frees it.
• Change the order of the methods in the script to match the order they are called, to make it easier to follow. They are now ordered as _on_load_pressed, _bg_load, _bg_load_done, _exit_tree.

Some of these changes are subjective, so let me know what you think. The last one is technically in violation of the GDScript style guide, since it puts a built-in virtual method after the private methods, but I think it improves readability enough to be worth it. I'm also not sure why the documentation says not to reuse thread objects, since doing so seems to work just fine.

[aaronfranke]

Looks good, thanks for this! And thanks for improving the engine too :)