feat: make timer stop game (Fixes #15)

app/src/main/java/sudoku/structure/MultiDoku.java

@@ -183,6 +183,15 @@ public class MultiDoku {
 		return emptyCells.get(randomIndex);
 	}
 
+	public void clearMutableCells() {
+		for (Sudoku s : getSubGrids()) {
+			for (Cell cell : s.getCells()) {
+				if (cell.isMutable())
+					cell.clearCurrentSymbol();
+			}
+		}
+	}
+
 	public MultiDoku clone() {
 		//TODO: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaah
 		return SudokuSerializer.deserializeSudoku(SudokuSerializer.serializeSudoku(this));