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.c

@@ -29,8 +29,10 @@
 static uint64_t next_handle = 1;
 
 HANDLE_IMPL(buf_T, buffer)
+HANDLE_IMPL(win_T, window)
 
 void handle_init()
 {
   HANDLE_INIT(buffer);
+  HANDLE_INIT(window);
 }

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);