feat: Add consistent Scene API with module-level properties (closes #151)

Replaces module-level scene functions with more Pythonic OO interface:

Scene class changes:
- Add `scene.children` property (replaces get_ui() method)
- Add `scene.on_key` getter/setter (matches on_click pattern)
- Remove get_ui() method

Module-level properties:
- Add `mcrfpy.current_scene` (getter returns Scene, setter activates)
- Add `mcrfpy.scenes` (read-only tuple of all Scene objects)

Implementation uses custom module type (McRFPyModuleType) inheriting
from PyModule_Type with tp_setattro for property assignment support.

New usage:
  scene = mcrfpy.Scene("game")
  mcrfpy.current_scene = scene
  scene.on_key = handler
  ui = scene.children

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/PySceneObject.h

@@ -27,7 +27,6 @@ public:
     
     // Scene methods
     static PyObject* activate(PySceneObject* self, PyObject* args);
-    static PyObject* get_ui(PySceneObject* self, PyObject* args);
     static PyObject* register_keyboard(PySceneObject* self, PyObject* args);
     
     // Properties