Fix #137: Game filter preferences are saved.

Game filter preferences are saved between runs of Cockatrice.

To test:
  * Start Cockatrice and select some filters.
  * Press Ok.
  * Restart Cockatrice and view the filters: the same filters are selected.

I decided to only save the filters when pressing Ok and not Cancel.
I was thinking about it and then I opted not to do it because I was afraid similarly named things might collide. I can add some restrictions if you can specify what those restrictions are. Maybe I could hash them?