patch 8.0.1596: no autocommand specifically for opening a terminal window

Problem:    No autocommand specifically for opening a terminal window.
Solution:   Add TerminalOpen. (?, closes #2484)
diff --git a/src/vim.h b/src/vim.h
index 3b458a0..86948b0 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -1346,6 +1346,7 @@
     EVENT_CMDUNDEFINED,		/* command undefined */
     EVENT_OPTIONSET,		/* option was set */
     EVENT_TEXTYANKPOST,		/* after some text was yanked */
+    EVENT_TERMINALOPEN,		/* after a terminal buffer was created */
     NUM_EVENTS			/* MUST be the last one */
 };