Implement handling of terminal focus events

Two new keys have been added to key_name_entry in keymap.c:
`FocusGained` and `FocusLost`.

Two cases have been added to the key handing switch in edit.c each
applying their respective autocmds.

In normal.c two functions have been added alongside nv_cursorhold doing
a similar job of applying the autocmd for the appropriate key.

tui/input.c has a new function `handle_focus_event` which eats either of
the control sequences for focus gained or lost. This function is checked
before handle_bracketed_paste and handle_forced_escape.

tui.c registers neovim as able to receive these control sequences in
terminfo_start and undoes that in terminfo_stop.

Closes #2302

src/nvim/keymap.c

@@ -284,6 +284,8 @@ static struct key_name_entry {
   {K_SNR,             (char_u *)"SNR"},
   {K_PLUG,            (char_u *)"Plug"},
   {K_PASTE,           (char_u *)"Paste"},
+  {K_FOCUSGAINED,     (char_u *)"FocusGained"},
+  {K_FOCUSLOST,       (char_u *)"FocusLost"},
   {0,                 NULL}
 };