conditional os in UI xml, add system dialog file open on osx, check api format or error message when opening a document, per-char text wrap on NodeText, additional info on create doc dialog like working path

src/app.h

@@ -94,6 +94,7 @@ public:
     struct engine* and_engine;
 #endif
     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 display_file(std::string path);
     void showKeyboard();
     void hideKeyboard();