Save game state and restore it from saved file

Add a new GameSave class to write the current game state into a game.sav file when the user leaves the app.
Reload the game state from startup function if a save file exists, create a new game otherwise.
Delete the save file after a completed game, if exists.

Tested manually :

• Exit the app using keyboard shortcut or app exit button before completing a game, then reload app to restore the game state.
• Exit the app from the score window, after completing a game.
• Exit the app after finishing a game and restarting a new one.

Feature Request #52 (closed)