patch 9.1.1391: Vim does not have a vertical tabpanel

Problem:  Vim does not have a tabpanel
Solution: include the tabpanel feature
          (Naruhiko Nishino, thinca)

closes: #17263

Co-authored-by: thinca <thinca@gmail.com>
Signed-off-by: Naruhiko Nishino <naru123456789@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/option.c

@@ -3107,6 +3107,9 @@ redraw_titles(void)
 {
     need_maketitle = TRUE;
     redraw_tabline = TRUE;
+#if defined(FEAT_TABPANEL)
+    redraw_tabpanel = TRUE;
+#endif
 }
 
 /*
@@ -8832,6 +8835,18 @@ option_set_callback_func(char_u *optval UNUSED, callback_T *optcb UNUSED)
 #endif
 }
 
+#if defined(FEAT_TABPANEL)
+/*
+ * Process the new 'showtabpanel' option value.
+ */
+    char *
+did_set_showtabpanel(optset_T *args)
+{
+    shell_new_columns();
+    return NULL;
+}
+#endif
+
 #if defined(FEAT_EVAL) || defined(PROTO)
     static void
 didset_options_sctx(int opt_flags, char **buf)