more general refactoring done

- renamed OpenedFieldsCount to OpenedCellsCount
- renamed ScanSurroundingFields to Scan(ScanSurroundingCells())
- renamed mines to minesCount(ScanSurroundingCells() method)
- renamed PrintAllFields() method to PrintGameBoardRevealed() (to keep
things consistent)
- added a Console.Clear() at each loop iteration - looks better (remove
later?)
- removed unnecessary 'ref' keyword from AddPlayerToScoreBoardMethod()