Add init() for PressStartPrompt
PressStartPrompt: separate timer for idle flash

On second thought, it will need one of these because the prompt will
need a way to differentiate between between being idle and having been
confirmed.
Have OptionList's update() handle confirmation

Instead of having flash_text() turn off the timer and call
respond_to_confirm(), update() will handle everything that has to do
with starting and ending the confirmation flash. Thanks to this,
PressStartPrompt can now use flash_text() for its idle flashing.

Additionally, update_text() was renamed back to update() to reflect
its added functionality as a result.
Change flash_text() to flash_confirmed_option()

This makes the method's specialized purpose more clear.
Of course, this means PressStartPrompt can't use it for the idle flash
animation after all; that will be handled in a separate method,
flash_idly().