feat: multi synced player scores

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

@@ -126,6 +126,8 @@ public class Cell {
 	}
 
 	public boolean trySetValue(int newValue) {
+		if (!isMutable())
+			return false;
 		if (!canHaveValue(newValue))
 			return false;
 		setSymbolIndex(newValue);

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

@@ -1,8 +1,10 @@
 package sudoku.structure;
 
-import java.util.*;
-
-import sudoku.io.SudokuSerializer;
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Random;
+import java.util.Set;
 
 /**
  * @class MultiDoku