Added set_mnemonic

[YLSDev]

Added creative island code support which was added ingame a while back 17.40

[Terbau]

This PR doesn't really offer anything new at all in this state. You only added a method to the PartyMeta class, which doesn't really let anyone interact with it. If I am to merge this you will have to do the following:

• Add a method to ClientParty so that you can interact with the meta method.
• Add thorough documentation to the method in ClientParty.

I am also wondering if this approach is really the best way to do it. Since mnemonic is a property of PlaylistData, it might be a better idea to just add a kwarg to PartyMeta.set_playlist() and also use this approach with ClientParty.set_playlist(). This would make it a bit more inline with how the library is already designed.

[YLSDev]

done