Remove references to TitleState from OptionList

...As well as its subclasses.
OptionList: Add is_animating()

TitleState will use this method to determine whether input should be
accepted.
OptionList: Add another branch to is_offscreen()

Of course, the OptionList shouldn't be considered 'offscreen' if it
is in the process of sliding in. As such, the list can only be in the
offscreen state at the end of the hide all animation.
OptionList: Automatically slide in when offscreen

OptionLists are smart: they know that they can't be of much use if the
players can't see them. Because of this, they will automatically set
their animation to 'show all' through update() if this is the case.

TitleState will prevent an infinite loop of sliding out and sliding in,
by immediately switching to the next OptionList before calling
update(). That way, the OptionList will be ready to slide in the next
time it is needed.