patch 8.1.0487: no menus specifically for the terminal window

Problem:    No menus specifically for the terminal window.
Solution:   Add :tlmenu. (Yee Cheng Chin, closes #3439) Add a menu test.
diff --git a/src/structs.h b/src/structs.h
index 560ce42..a0c06b5 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -3101,8 +3101,9 @@
 #define MENU_INDEX_OP_PENDING	3
 #define MENU_INDEX_INSERT	4
 #define MENU_INDEX_CMDLINE	5
-#define MENU_INDEX_TIP		6
-#define MENU_MODES		7
+#define MENU_INDEX_TERMINAL	6
+#define MENU_INDEX_TIP		7
+#define MENU_MODES		8
 
 /* Menu modes */
 #define MENU_NORMAL_MODE	(1 << MENU_INDEX_NORMAL)
@@ -3111,6 +3112,7 @@
 #define MENU_OP_PENDING_MODE	(1 << MENU_INDEX_OP_PENDING)
 #define MENU_INSERT_MODE	(1 << MENU_INDEX_INSERT)
 #define MENU_CMDLINE_MODE	(1 << MENU_INDEX_CMDLINE)
+#define MENU_TERMINAL_MODE	(1 << MENU_INDEX_TERMINAL)
 #define MENU_TIP_MODE		(1 << MENU_INDEX_TIP)
 #define MENU_ALL_MODES		((1 << MENU_INDEX_TIP) - 1)
 /*note MENU_INDEX_TIP is not a 'real' mode*/