API: Refactor: Register/unregister created/destroyed tabpages

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

src/nvim/api/private/handle.h

@@ -11,6 +11,7 @@
 
 HANDLE_DECLS(buf_T, buffer)
 HANDLE_DECLS(win_T, window)
+HANDLE_DECLS(tabpage_T, tabpage)
 
 void handle_init(void);