We have just changed the setting to let a skill to be learned(upgraded) multiple times. So change the mechanism to the following:
Every time exp incr and meets the next integer(that is, able to get to a new skill level), ignore any further attempts to incr skill exp, and enable skill learning in GUI.(AbilityData should provide a method canSkillUpgrade(int)).
Also for wrapping purpose we should provide a getSkillLevel(int sid) function doing the floor internally.

Created field of skill level in AbilityData. 337869e.
Now it is the Category that is responsible to handle skill expr change event. Maybe we should make an api in SkillBase.

Added in 63a5629.