Make the ordering of the backgrounds more consistent

[haroldo-ok]

One other thing I'd just like to point out. Sometimes I set the backgrounds with a numeric variable instead of a name. On different occasions, the first background is number 0, and on other occasions, the first background is number 3.
https://itch.io/post/14878055

[NickR-Git]

Not sure this is necessarily a bug?

If a user create 4 backgrounds (0-3), then deletes the first 3 (0-2), to not break project functionality it makes sense that background 3 remains background 3, otherwise when setting backgrounds by # ID in blocks, those IDs would need to be either automatically or manually checked and updated whenever a background is created or removed.