fix some ui glitch with NodeButton, start implementing working dir option

src/app_events.cpp

@@ -90,6 +90,27 @@ void App::pick_file(std::vector<std::string> types, std::function<void (std::str
 #endif
 }
 
+void App::pick_dir(std::function<void(std::string path)> callback)
+{
+    redraw = true;
+#ifdef __IOS__
+    // NOT IMPLEMENTED
+#elif __OSX__
+    dispatch_async(dispatch_get_main_queue(), ^{
+        std::string path = [osx_view pick_dir];
+        if (!path.empty())
+            callback(path);
+    });
+#elif __ANDROID__
+    // NOT IMPLEMENTED
+#elif _WIN32
+    // TODO: to be implemented
+//    std::string path = win32_open_file();
+//    if (!path.empty())
+//        callback(path);
+#endif
+}
+
 void App::display_file(std::string path)
 {
 #ifdef __IOS__