new friendship system

It's a simple system for sharing a world with a friend or a group of friends. There's a new setting in the world creation process where you configure this, and you can disable it if you don't want to share the world with anyone. It works like this: when you finish modifying your world, you go to the tab and invite a friend from your friends list to create this world together. On that player's screen, it will show "Your friend invited you to create a world together," and they will have the option to accept or decline. If the friend accepts, they can access and modify the world, like activating cheats, etc.