Fix C++ event window lookup and Linux WebKit navigation handling; update serve_a_folder example

[triuk]

Summary

This PR includes three follow-up changes after the member-function bind support:

1. C++ API fix for the b36c04e:

   • event::get_window() now resolves the window using bind_id (not window), preventing invalid callback-window lookups.

2. Linux/WebKit navigation fix:

   • webui_navigate() Linux WebView path now goes through webui_show(window, url) to avoid navigation re-entrancy crashes when called from all-events navigation handlers.

3. Example updates:

   • examples/C++/serve_a_folder was refactored to a class-based style using member-function binding.
   • Added serve_a_folder to CMake examples target registration.
   • Updated C++ examples README accordingly.

Validation

• Built successfully in debug build (serve_a_folder target).
• Verified that:
  • OpenNewWindow works,
  • SwitchToSecondPage works,
  • link navigation from the page no longer crashes in the tested Linux/WebKit setup.
• I also verified examples on Windows (Chromium)
• I did not verify on macOS.

[AlbertShown]

Thank you @triuk, useful contribution