event: No longer process K_EVENT automatically

Two new functions, `event_enable_deferred()`/`event_disable_deferred()` have to
be called by code that is capable of handling asynchronicity. User-dialog states
like "press ENTER to continue" or the swap file confirmation no longer will
generate K_EVENT.

src/nvim/getchar.c

@@ -2481,7 +2481,6 @@ inchar (
       char_u dum[DUM_LEN + 1];
 
       for (;; ) {
-        event_process();
         len = ui_inchar(dum, DUM_LEN, 0L, 0);
         if (len == 0 || (len == 1 && dum[0] == 3))
           break;