All pointers held by bonus itself are now const.
To support OppositeSideLimiter (the only stateful limiter) bonuses now
hold their player owner instead.
No changes in functionality or mods
- Removed 'version' field from serialize() method
- Handler classes - Binary(De)Serializer now have 'version' field
- Serialization versioning now uses named enum
Save compatibility with 1.4.X saves should be intact
