Add API and implementation for adding and removing MV worlds without loading/unloading.

[cybertiger]

Other plugins may wish to add MV worlds which they have loading using a custom world loader to the MV config, and later remove them.

They should be able to do this without causing Mutliverse to attempt to load them.

[cybertiger]

Leaving this open, but I don't need it anymore, and it turns out it doesn't do what I need anyway.

I forgot you can't configure a multiverse world if it's not loaded - fail.

[cybertiger]

Further comment, it turns out some of this patch is useful.

Being able to remove a world from the config if it's not loaded is really useful!

[fernferret]

This is 9 months old, is this still useful? Do we still want it?

[dumptruckman]

Perhaps it could be updated to work with latest code base?

[benwoo1110]

outdated PR