patch 8.1.2044: no easy way to process postponed work

Problem:    No easy way to process postponed work. (Paul Jolly)
Solution:   Add the SafeState autocommand event.

src/vim.h

@@ -1315,6 +1315,7 @@ enum auto_event
     EVENT_QUICKFIXCMDPRE,	// before :make, :grep etc.
     EVENT_QUITPRE,		// before :quit
     EVENT_REMOTEREPLY,		// upon string reception from a remote vim
+    EVENT_SAFESTATE,		// going to wait for a character
     EVENT_SESSIONLOADPOST,	// after loading a session file
     EVENT_SHELLCMDPOST,		// after ":!cmd"
     EVENT_SHELLFILTERPOST,	// after ":1,2!cmd", ":w !cmd", ":r !cmd".