Begin writing the BattleSetupList class
Create the BattleSetupIndex enum
BattleSetupList: Add init(), create_options()
UML: Add cancel_setup() to BattleSetupList

The operation COULD be implemented directly within handle_input(),
but turning it into a separate method provides more consistency as well
as better separation between operations.
BattleSetupList: Add handle_input() + support
OptionList: Disable input handling when animating

Obviously, the game shouldn't be reading player input while flashing
an Option or hiding/showing an OptionList.