Cvar: start upgrading the cvar system

* Many things are broken
  * The flags CHEAT, ROM, SHADER, ... do nothing
  * listCvars is temporarily removed
  * other things?
* Each upgraded cvar contains an old cvar which is synchronized by some
code in CvarSystem.h
* Cvar.h is basically a forward declaration of CvarProxy (none are
instanciated), it will be rewritten.