add changelog dialog and display_file api (implemented in osx and ios)

src/node_changelog.h

@@ -0,0 +1,13 @@
+#pragma once
+#include "node.h"
+#include "node_button.h"
+
+class NodeChangelog : public Node
+{
+    Node* m_template;
+    NodeButton* btn_ok;
+public:
+    virtual Node* clone_instantiate() const override;
+    virtual void init() override;
+    virtual kEventResult handle_event(Event* e) override;
+};