Add (.png)image for set cards.

- Add back and selected image for Set cards.
- Re-Write <backgroundImageForCard> in subclass for Set cards.
Remove UI related code to controller.

- Make sure model does ONLY UI independent code.
- Controller is responsible for put model data into UI.
Remove/Modify UI related code from model

- Modify the symbols to regular string, not UI symbols.
Modify SetCard controller to UI

- re-write setTitleOfCard.