CharacterPreview: Add calculate_y_position

Just to make the init method a little less cluttered.
CharacterPreview: Write correct_position()
CharacterSelectState: Begin writing init()

More will be added to this method as its supporting methods are being
created.
Rename is_reversed attribute to is_facing_left

More descriptive, wouldn't you say?
CharacterPreview: Add get_num_of_frames()
CharacterPreview: Fix get_num_of_frames()

No need to subtract 1 since we're talking about length, not index.
CharacterPreview: Add flip_sprite() + support