New: sessionStorage and infinite state save option

- The stateDuration option has been updated to provide the ability to
  indicate if sessionStorage should be used rather than localStorage
  (set value to -1).

- Settings stateDuration to 0 also indicates that the duration is
  infinity.

- Parts of the state saving have been optimised for code size, so this
  commit actually reduces the min size by 32 bytes despite the new
  abilities.

- See thread 19572 for discussion on this