Guide examples should open a new tab rather than replace the existing code

I've seen my child make this error a few times, whereby he'll click an example in the guide and it will overwrite what is in the currently opened CodeWorld file. Today he somehow ended up permanently (as far as I can tell) deleting the work he had in an existing file. It seems like it may be more desirable to either create a new file when the user interacts with an example in the guide, or append what is in the example to the currently open file.