implemented worldname-check

[EinDev]

should there be a limit for world name size?

[nagalun]

Hey TimonMi, thanks for your contribution.
Late reply, but indeed, world names should be at least 1 character and not more than 24.
To avoid confusion and problems with some filesystems world names are always lowercase, it's the client's job to send the world name it wants to join in lowercase, that's why A-Z isn't checked in the for loop.