Added a handy callback listener for UI

src/UiCallbackListener.h

@@ -0,0 +1,22 @@
+#pragma once
+#include <RmlUi/Core/Event.h>
+#include <RmlUi/Core/EventListener.h>
+#include <RmlUi/Core/Element.h>
+#include <functional>
+
+class UiCallbackListener : public Rml::EventListener {
+public:
+    using Callback = std::function<void(Rml::Event&)>;
+    explicit UiCallbackListener(Callback callback) : callback(std::move(callback)) {}
+
+    void ProcessEvent(Rml::Event& event) override {
+        if (callback) callback(event);
+    }
+
+    void OnDetach(Rml::Element* /*element*/) override {
+        delete this;
+    }
+
+private:
+    Callback callback;
+};