First feebacks patch

Since the launch of the game, the first feedbacks arrived so I made some minors changes accordingly :

• Added an option to show the game windowed

• Options are now saved and will be remembered when exiting the game

• Some players confused the ruby sprite for a heart, so the sprite has been modified to make the distinction clearer

• Added a visual feedback on the cursor when the attack is ready

• The button "Continue" is now at the top of the main menu when available instead of "New game"