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

src/engine/Time.hpp

@@ -0,0 +1,35 @@
+#pragma once
+
+#include <stdint.h>
+
+class Time {
+    uint64_t frame = 0;
+    double lastTime = 0.0;
+    double delta = 0.0;
+public:
+    Time() {}
+
+    void update(double currentTime) {
+        frame++;
+        delta = currentTime - lastTime;
+        lastTime = currentTime;
+    }
+
+    void step(double delta) {
+        frame++;
+        lastTime += delta;
+        this->delta = delta;
+    }
+
+    void set(double currentTime) {
+        lastTime = currentTime;
+    }
+
+    double getDelta() const {
+        return delta;
+    }
+
+    double getTime() const {
+        return lastTime;
+    }
+};