Several things: see description

- Add property IsMine in abstract class Cell
- Remove IsDetonated and Detonate from Mine: doubling functionality with
Exploded/Explode
- Remove Position, X and Y from Cell: the array in Gameboard has the
same functionality
- Move the repeating code from the 'detonation pattern' classes to their
base
- Rename FieldGenerator to GameboardGenerator and make it return
instance of Gameboard (instead of Cell[,]). Set the gameboard number of
mines and size in the generator
- In the code work with Gameboard.Size instead of
Gameboard.Field.GetLength(..)
- Organize folders