Added constant representing even number

We need to know if some numbers (like number of rows) are odd or even to
find out if it is solvable
Added function isSolvable, moved struct GameState

I moved structure GameState because it was needed in the other file.
Function isSolvable return 1 if this state is solvable
Added use of isSolvable and error message
Added check for bad input

Whenever any number in input is used more than once, than input is
incorrect (program return 2)