New level progression

I added a new level (#5) to hopefully introduce a mechanic more naturally. Thanks to everyone who struggled while I watched unhelpfully, it wasn't for nothing.

I also removed the third tutorial (CCCC), which I think was only causing early confusion. The intention was to show that you can move by dragging anywhere, but I don't think this is essential knowledge, and it also got caught up with introducing overlap.

For anyone who's finished the puzzle before, there's nothing new, but I'm giving the end game some more thought!

I've tagged this alpha 6, for reference the version that had been online since May 2022 was alpha 5.