OptionList: Replace next_list with next_state

And remove all references to next_list.
Use next_state in OptionList's subclasses
Fix a bug in OptionList's is_offscreen()

Adding a little more than we need there.
OptionList: Increase TEXT_SLIDE_SPEED

A bit faster makes it much cooler. Wooosh!
PressStartPrompt: Make prompt visible on press

Before this commit, the prompt would remain invisible during the slide
out animation if the players pressed Start while the visibility was
toggled off.