API: Refactor: Register/unregister created/destroyed windows

- Add the 'handle' field to `win_T`
- Add declare/implement functions for registering/unregistering/retrieving
  windows
- Register/unregister windows when they are created/destroyed.

src/nvim/api/private/handle.h

@@ -10,6 +10,7 @@
   void handle_unregister_##name(type *name);
 
 HANDLE_DECLS(buf_T, buffer)
+HANDLE_DECLS(win_T, window)
 
 void handle_init(void);