New achievements system

Implements
https://github.com/UndercastTeam/UndercastClient/commit/ba4c4cb9fbfba9472a0c35f3a2d6bfbccee275c3
for ModLoader.

But this commit does not remove the old system because it is still
needed for enabling ModLoader
(https://github.com/UndercastTeam/UndercastClient/commit/d506dcda0d688c2848693505e73a2a6467fe5633)