Create a new file for the Character Select State
UML: Add additional methods for player input

get_current_player() determines which player is currently choosing
based on whether p1_char_index has been given a value other than None.
The value it returns will then be passed to get_input_name() to
determine which player's key bindings should be checked.
UML: Rename fix_position() to correct_position()

Done for better clarity, since 'fix' could be misinterpreted as meaning
to keep the object in one position.
Begin writing the CharacterPreview class