add class-based call_js_from_cpp example and member-function bind overload

[triuk]

Adds ergonomic class-member binding in the C++ API and introduces a class-based example.
The previous callback signature made C++ class usage too complex.
Now you can do just this in the constructor and you are good to go:

my_window.bind("my_function_count", this, &CounterApp::my_function_count);

This update enables direct, readable binding of member functions without workarounds. Just like in the Python-WebUI.

Changes

• include/webui.hpp

  • switched callback storage in window::event::handler to std::function<void(event*)> (from raw function pointers)
  • updated internal callback bookkeeping to map + mutex
  • added overload: bind(element, instance, &Class::method)

• examples/C++/call_js_from_cpp_class/main.cpp

  • new example equivalent to call_js_from_cpp, but implemented with a class

• CMakeLists.txt

  • added call_js_from_cpp_class target

• examples/C++/README.md

  • listed the new example

[AlbertShown]

Thank you @triuk for this PR.
C to C++ class method calls was in the todo list for a while.
Thank you for fixing this, very useful.

After marge, I will replace the old example call_js_from_cpp with this new one.

[triuk]

@AlbertShown Hi, I just found serve_a_folder example, which uses class by default. I did not notice it as it was not a build target in CMakeFiles.txt.
I tried to rewrite it with the new class bind to have up-to-date code, but I found a bug. I am on it, probably solved. I will do another PR after more testing.

[AlbertShown]

Sounds good @triuk, Thank you.