New game data keys: playable and recommended

[nikuda]

playable:Boolean = if the game can be played, currently 'playable' string is placed either in the development status keys or in the info key.

recommended:Boolean = games of very high quality/playability, determines what gets rendered in the position that ScummVM occupies now.

[nikuda]

Ah already talked about in #30, agree with everything in this comment #30 (comment).

[wonea]

I'm not sure about recommended, but playable: would be a good start.

[piranha]

I'd say if we can come up with criteria for recommended, then it would be nice. If we can't, I would rather leave ScummVM in place just because it implements so many games and that's it.

playable sounds good!