implement clipboard on iOS

src/app.h

@@ -144,6 +144,8 @@ public:
     struct android_app* and_app;
     struct engine* and_engine;
 #endif
+    std::string clipboard_get_text();
+    bool clipboard_set_text(const std::string& s);
     void pick_image(std::function<void(std::string path)> callback);
     void pick_file(std::vector<std::string> types, std::function<void(std::string path)> callback);
     void pick_dir(std::function<void(std::string path)> callback);