rename engine.hpp/cpp to Engine.hpp/cpp & move Engine, Time and mainloop classes to 'engine' folder

src/engine/ServerMainloop.hpp

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <memory>
+
+class Level;
+class LevelController;
+class Engine;
+
+class ServerMainloop {
+    Engine& engine;
+    std::unique_ptr<LevelController> controller;
+public:
+    ServerMainloop(Engine& engine);
+    ~ServerMainloop();
+
+    void run();
+
+    void setLevel(std::unique_ptr<Level> level);
+};