Initial refactoring: renaming mainly

In class Mine add private fields for coordinates.
In class Mine add constructor with 2 parameters and substitute all
callings to new Mine().
Name constants accordingly.
Substitute symbols '-' and 'X' with a char constant.
Refactor method Contains: use .Net method List.Contains.
Rename VPoletoLiE to IsInsideField, reafactor the code.
Refactor methods ExplodeOne and ExplodeThree: do not create Mine, since
it is used only to get coordinates - work directly with coordinates.
Refactor method IsValidMove: put the nominal case (true) at the
beginning.
Rename PokajimiRezulta to ShowResult.