Flashing potions

Starting to fix #1796

Player will flash potion colour while potion is in effect. For health/money potions, they flash for one second. I'm not entirely certain what happens to the timer when you either select another potion or get hurt. It should cancel the timer & start flashing the new colour but I'm not confident about that.

To get potions from the command line, use "-x give_potions" or from the cheat screen "no no juice".