Reworked the command registration system

Please read the [javadoc](https://github.com/Limeth/Terasology/blob/16965803d4edd128ce5baaa3c35e814de26752f3/engine/src/main/java/org/terasology/logic/console/Command.java).
Why is it called ClientCommands? Two of the command have the flag runOnServer = true

Maybe you want to add a sentence to the javadoc what kind of commands should go in this class.