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.
Are there character restrictions on this? Since the values are set by the server it might do something bad with some names. Especially on windows where it's stored in the registry instead of an ini