Add max level of 20

[jomibauer]

• adjust ExperienceManager and LevelComponent to have a max level of 20
• adjust AwardExpState and ExperienceViewController to handle units of level 20 (no exp)
• adjust AwardExpState and ExperienceViewController to handle units that level up to 20 (level up, then ignore remaining exp)
• fix LevelUpPane to change lvlText when playing a level up