vim-patch:8.1.1300: in a terminal 'ballooneval' does not work right away

Problem:    In a terminal 'ballooneval' does not work right away.
Solution:   Flush output after drawing the balloon.  Add the <Ignore> key
            code.  Add a test.
2f10658b06

src/nvim/keymap.c

@@ -309,6 +309,7 @@ static const struct key_name_entry {
   { K_ZERO,            "Nul" },
   { K_SNR,             "SNR" },
   { K_PLUG,            "Plug" },
+  { K_IGNORE,          "Ignore" },
   { K_COMMAND,         "Cmd" },
   { 0,                 NULL }
   // NOTE: When adding a long name update MAX_KEY_NAME_LEN.