patch 8.1.1537: using "tab" for popup window can be confusing

Problem:    Using "tab" for popup window can be confusing.
Solution:   Use "tabpage". (Hirohito Higashi, closes #4532)
diff --git a/src/popupwin.c b/src/popupwin.c
index 5265b4b..594fca8 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -748,8 +748,8 @@
     // Avoid that 'buftype' is reset when this buffer is entered.
     buf->b_p_initialized = TRUE;
 
-    if (dict_find(d, (char_u *)"tab", -1) != NULL)
-	nr = (int)dict_get_number(d, (char_u *)"tab");
+    if (dict_find(d, (char_u *)"tabpage", -1) != NULL)
+	nr = (int)dict_get_number(d, (char_u *)"tabpage");
     else if (type == TYPE_NOTIFICATION)
 	nr = -1;  // notifications are global by default
     else
@@ -757,7 +757,7 @@
 
     if (nr == 0)
     {
-	// popup on current tab
+	// popup on current tab page
 	wp->w_next = curtab->tp_first_popupwin;
 	curtab->tp_first_popupwin = wp;
     }
@@ -1228,7 +1228,7 @@
 
 /*
  * Reset all the POPF_HANDLED flags in global popup windows and popup windows
- * in the current tab.
+ * in the current tab page.
  */
     void
 popup_reset_handled()