Use IEnumerable instead of array of cells to respect incapsulation
Use cell type to check if the cell was used or not.
Cell is basically just a string. Update Cell with string type. Board will now just expose cell related operations and will be used by Logic.
Since cell is a string, SymbolType and SymbolTypeConverter are no longer required
Move config from app.config to Config class because we only need to implement TicTacToe.

Tic Tac Toe is 3x3 grid game. Since the game did not automatically
update its logic based on the config values because it was not required,
it's wrong to expose config values. Therefore use an internal class for
configuration.
Minor refactoring