[BREAKING] Removed Client-only mode for addons

- Addons can now be SERVER or BOTH. This does not mean there isn't
  "client-only" addons, it simply means that on the client there is always
  a server so the CLIENT mode option made no sense. Specifiying SERVER means the
  addon will exist only on an external server.
- Changed LanguageRegistry to LanguageManager (matching the AddonManager)
- Split handling from Message. Allows a developer to have a seperate
  handling class (implementing MessageHandler) for very complex messages.

Signed-off-by: Chris Sanders <zidane@outlook.com>