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 mean are there restrictions imposed by the registry/INI format. Like if I send you obnoxious Unicode format names from the server, will this break? Hashing would be an OK solution I think, just pick something fast. Nothing realistic will collide and even if it did the impact isn't large