Save game when app looses focus

[taratatach]

Related to #11 (in French).

We need to save the state of the current game whenever the app looses focus so the player doesn't loose the current game.

A future improvement of this could be the ability to save multiple games.

[Flaburgan]

Are you sure the game is lost if focus is lost? Using Android, when the focus is lost but the application is not exited, the game is still there (example: you receive an SMS, go in your sms app, read it and then close the sms app, FFDoku will be there and the game won't be lost.)

[taratatach]

We shouldn't assume the state of data when loosing focus and killing the app will definitely loose the game.