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/ex_docmd.c b/src/ex_docmd.c
index 70acfc8..d00ca56 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -4283,6 +4283,7 @@
 	case CMD_omenu:	    case CMD_onoremenu:	    case CMD_ounmenu:
 	case CMD_imenu:	    case CMD_inoremenu:	    case CMD_iunmenu:
 	case CMD_cmenu:	    case CMD_cnoremenu:	    case CMD_cunmenu:
+	case CMD_tlmenu:    case CMD_tlnoremenu:    case CMD_tlunmenu:
 	case CMD_tmenu:				    case CMD_tunmenu:
 	case CMD_popup:	    case CMD_tearoff:	    case CMD_emenu:
 	    return set_context_in_menu_cmd(xp, cmd, arg, forceit);