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/runtime/menu.vim b/runtime/menu.vim
index 5e4bd1f..a6ecf3c 100644
--- a/runtime/menu.vim
+++ b/runtime/menu.vim
@@ -160,6 +160,9 @@
 cnoremenu 20.350 &Edit.&Copy<Tab>"+y		<C-Y>
 nnoremenu 20.360 &Edit.&Paste<Tab>"+gP		"+gP
 cnoremenu	 &Edit.&Paste<Tab>"+gP		<C-R>+
+if exists(':tlmenu')
+  tlnoremenu	 &Edit.&Paste<Tab>"+gP		<C-W>"+
+endif
 exe 'vnoremenu <script> &Edit.&Paste<Tab>"+gP	' . paste#paste_cmd['v']
 exe 'inoremenu <script> &Edit.&Paste<Tab>"+gP	' . paste#paste_cmd['i']
 nnoremenu 20.370 &Edit.Put\ &Before<Tab>[p	[p