feat: add history

app/src/main/java/sudoku/solver/SolverStep.java

@@ -0,0 +1,23 @@
+package sudoku.solver;
+
+import sudoku.structure.Cell;
+
+public class SolverStep {
+
+	private final Cell cell;
+	private final int newValue;
+
+	public SolverStep(Cell cell) {
+		this.cell = cell;
+		this.newValue = cell.getSymbolIndex();
+	}
+
+	public int getNewValue() {
+		return newValue;
+	}
+
+	public Cell getCell() {
+		return cell;
+	}
+
+}