Added a base class for score cells.
Added class ScoreColumn that stores the cells and cell names of a column.
Added a player class.
Created class GameModel which provides a simple interface to the game state.

A simple implementation of DiceModel was added to get source for die values.
Added a cell type that calculates the sum of the given kind of dice.
Added support for non-markable cells.
Removed overcomplicated score interface from GameModel.
Use immutable lists instead of arrays in GameModel and ScoreColumn.
Better player retrieval in GameModel.
Implemented player turns.
Added implementations for different score cells.
Added GameController and GameView.
Avoid a null pointer exception in MarkableScoreCell.setPlayer.