Use unused sounds

Supersedes !13 (closed)

Now we use sounds that were unused by KDiamond instead of relying on sounds from the sound theme.

I was a bit torn about using KDiamond-Stone-Touch.ogg for the game over screen (I guess it was meant for another action), but I decided to use it because 1. It's consistent with the other sounds and it surprisingly doesn't sound out of place 2. Not relying on an external sound theme means we're sure to always get a sound, especially considering how most sound themes for Linux (https://www.gnome-look.org/browse?cat=316&ord=rating) don't include a game-over-winner sound. If that choice is not okay we could instead use a more common sound, like dialog-information.

@aacid @kossebau