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

src/engine/Mainloop.cpp

@@ -0,0 +1,43 @@
+#include "Mainloop.hpp"
+
+#include "Engine.hpp"
+#include "debug/Logger.hpp"
+#include "frontend/screens/MenuScreen.hpp"
+#include "frontend/screens/LevelScreen.hpp"
+#include "window/Window.hpp"
+#include "world/Level.hpp"
+
+static debug::Logger logger("mainloop");
+
+Mainloop::Mainloop(Engine& engine) : engine(engine) {
+}
+
+void Mainloop::run() {
+    auto& time = engine.getTime();
+
+    engine.setLevelConsumer([this](auto level) {
+        if (level == nullptr) {
+            // destroy LevelScreen and run quit callbacks
+            engine.setScreen(nullptr);
+            // create and go to menu screen
+            engine.setScreen(std::make_shared<MenuScreen>(engine));
+        } else {
+            engine.setScreen(std::make_shared<LevelScreen>(engine, std::move(level)));
+        }
+    });
+
+    logger.info() << "starting menu screen";
+    engine.setScreen(std::make_shared<MenuScreen>(engine));
+    
+    logger.info() << "main loop started";
+    while (!Window::isShouldClose()){
+        time.update(Window::time());
+        engine.updateFrontend();
+        if (!Window::isIconified()) {
+            engine.renderFrame();
+        }
+        engine.postUpdate();
+        engine.nextFrame();
+    }
+    logger.info() << "main loop stopped";
+}