Added support for sacrifice to UnlessCost

I added the ability to use sacrifice costs for an UnlessCost. I only converted Rathi Dragon to the new capability because I wanted to make sure everyone was ok with implementation. I changed how the input works for choosing the cards to sacrifice. Clicking a card once adds it to the sacrifice list. Clicking a card again removes it from the list. The selected cards are displayed in the prompt panel. Once the required number of cards are chosen, the OK button gets enabled. The cards are only sacrificed after the OK button is pressed so the user can change or cancel their choices without removing the cards permanently.