feat/refactor: actually track win conditions

src/main/java/uulm/teamname/marvelous/gamelibrary/entities/Inventory.java

@@ -40,6 +40,11 @@ public class Inventory implements Iterable<StoneType> {
         return size - content.size();
     }
 
+    /** Returns the number stones the inventory has. */
+    public int getSize() {
+        return content.size();
+    }
+
     /**
      * Checks if the inventory contains the given stone.
      * @param stone The {@link StoneType} to check for