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.c

@@ -30,9 +30,11 @@ static uint64_t next_handle = 1;
 
 HANDLE_IMPL(buf_T, buffer)
 HANDLE_IMPL(win_T, window)
+HANDLE_IMPL(tabpage_T, tabpage)
 
 void handle_init()
 {
   HANDLE_INIT(buffer);
   HANDLE_INIT(window);
+  HANDLE_INIT(tabpage);
 }

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);
 

src/nvim/buffer_defs.h

@@ -738,6 +738,7 @@ struct diffblock_S {
  */
 typedef struct tabpage_S tabpage_T;
 struct tabpage_S {
+  uint64_t handle;
   tabpage_T       *tp_next;         /* next tabpage or NULL */
   frame_T         *tp_topframe;     /* topframe for the windows */
   win_T           *tp_curwin;       /* current window in this Tab page */

src/nvim/window.c

@@ -2845,6 +2845,7 @@ void win_init_size(void)
 static tabpage_T *alloc_tabpage(void)
 {
   tabpage_T *tp = xcalloc(1, sizeof(tabpage_T));
+  handle_register_tabpage(tp);
 
   /* init t: variables */
   tp->tp_vars = dict_alloc();
@@ -2859,6 +2860,7 @@ void free_tabpage(tabpage_T *tp)
 {
   int idx;
 
+  handle_unregister_tabpage(tp);
   diff_clear(tp);
   for (idx = 0; idx < SNAP_COUNT; ++idx)
     clear_snapshot(tp, idx);