Create scroll value methods for BattleSetting
Create highlight methods in BattleSetting
Fix a mistake in BattleSetting's scroll methods

Remember: selected_value refers to the index of the value, not the
actual value itself.
BattleState: rename selected_value to value_index

Its purpose is clearer this way.
BattleState: create get_value() method

This will make a simple task a little less verbose later on.