Detect and handle blocked positions

[ipkiss42]

There are 2 types of blocked positions:

1. The rack does not allow to play any word on the current board. This can be easily detected by performing a search, but it needs to be handled properly (do we stop the game? what if it was the first turn?)
2. No rack that can be drawn with the rack allows to play a word. This is much harder to detect...

Fortunately, blocked positions are quite rare in practice, so the priority is not too high.