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

src/engine/PostRunnables.hpp

@@ -0,0 +1,29 @@
+#pragma once
+
+#include <queue>
+#include <mutex>
+#include "delegates.hpp"
+
+class PostRunnables {
+    std::queue<runnable> runnables;
+    std::recursive_mutex mutex;
+public:
+    void postRunnable(runnable task) {
+        std::lock_guard<std::recursive_mutex> lock(mutex);
+        runnables.push(std::move(task));
+    }
+
+    void run() {
+        std::queue<runnable> tasksToRun;
+        {
+            std::lock_guard<std::recursive_mutex> lock(mutex);
+            std::swap(tasksToRun, runnables);
+        }
+
+        while (!tasksToRun.empty()) {
+            auto& task = tasksToRun.front();
+            task();
+            tasksToRun.pop();
+        }
+    }
+};