Get levels for specified class

[osipxd]

LevelSystem knows nothing about classes but commonly user can have more than one class and each class can have own leveling.

For example, in Heroes player can have a class and a profession. In some cases player can have even more than two classes.

Solution requirements:

• Allow to get levels/experience for specified class
• Make it simple to implement LevelSystem and ClassSystem for the case when player have more than one class
• Player can have classes with levels (ex. class, profession) or without levels (ex. race)
• Keep LevelSystem and ClassSystem simple for the case when player can have only one class
• Make it backward compatible if possible

[osipxd]

Proposal from @NeumiumTo:

could be fixed by adding class name, or type like Primary /Secondary to the setExp and getExp methods

[osipxd]

My thoughts:
Maybe it will be better to return some PlayerClass that will implement LevelSystem itself.
But such class system will be harder to implement.
On other hand, PlayerClass is a good extension point to make classes more feature-rich.