Monkey Patch support + Robust callback tracking

McRogueFace needs to accept callable objects (properties on C++ objects)
and also support subclassing (getattr on user objects). Only direct
properties were supported previously, now shadowing a callback by name
will allow custom objects to "just work".
- Added CallbackCache struct and is_python_subclass flag to UIDrawable.h
- Created metaclass for tracking class-level callback changes
- Updated all UI type init functions to detect subclasses
- Modified PyScene.cpp event dispatch to try subclass methods

src/UISprite.cpp

@@ -7,7 +7,8 @@
 
 UIDrawable* UISprite::click_at(sf::Vector2f point)
 {
-    if (click_callable)
+    // #184: Also check for Python subclass (might have on_click method)
+    if (click_callable || is_python_subclass)
     {
         if(sprite.getGlobalBounds().contains(point)) return this;
     }
@@ -533,6 +534,13 @@ int UISprite::init(PyUISpriteObject* self, PyObject* args, PyObject* kwds)
         }
     }
 
+    // #184: Check if this is a Python subclass (for callback method support)
+    PyObject* sprite_type = PyObject_GetAttrString(McRFPy_API::mcrf_module, "Sprite");
+    if (sprite_type) {
+        self->data->is_python_subclass = (PyObject*)Py_TYPE(self) != sprite_type;
+        Py_DECREF(sprite_type);
+    }
+
     return 0;
 }