Practice: turn eval-based chapters to preview chapters

[dav1312]

Recently with the upgrade to SF 16 a lot of practice studies have been broken due to the evaluation not being the same as before.

While some practice studies understandably use the engine to allow the user to play any move as long as its good enough, some of them are clearly meant to have just 1 solution so there is no need to check the engine evaluation at all.

Some practice studies even have multiple solutions that do not correspond to the topic of the practice, like this one. Here the point is for black to take the rook, but the engine says Qe7 is even better.