Added level selection function

Originally there were progress saves, both for coins and the player's warehouse, but there were no saves for level progress because I started out planning to do random levels and wanted to categorise the levels by difficulty, so the player will play three levels of easy random, then start with three levels of random medium, then one level of random high difficulty, and then start the rounds over again. I didn't finish it, so when the player died, they would start over and start from the first level again. But now I've decided to show the levels in order. If you randomise the levels, you'll never get to play many of them, and then the player encounters repeated levels again.

Now with the level saving feature, you can select the level you've played directly from the main menu.