refactor: change pointer parameters to references for Level and Content in various classes

src/objects/Entities.hpp

@@ -166,7 +166,7 @@ public:
 
 class Entities {
     entt::registry registry;
-    Level* level;
+    Level& level;
     std::unordered_map<entityid_t, entt::entity> entities;
     std::unordered_map<entt::entity, entityid_t> uids;
     entityid_t nextID = 1;
@@ -184,7 +184,7 @@ public:
         float distance;
     };
 
-    Entities(Level* level);
+    Entities(Level& level);
 
     void clean();
     void updatePhysics(float delta);