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

src/app.h

@@ -94,6 +94,7 @@ public:
     struct engine* and_engine;
 #endif
     void pick_image(std::function<void(std::string path)> callback);
+    void display_file(std::string path);
     void showKeyboard();
     void hideKeyboard();
     void initLog();
@@ -139,6 +140,7 @@ public:
     void init_menu_layer();
     void init_menu_timelapse();
     void init_menu_about();
+    void dialog_changelog();
     void dialog_about();
     void dialog_newdoc();
     void dialog_save();