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/edit.c

@@ -593,9 +593,17 @@ edit (
      * Get a character for Insert mode.  Ignore K_IGNORE.
      */
     lastc = c;                          /* remember previous char for CTRL-D */
+    event_enable_deferred();
     do {
       c = safe_vgetc();
     } while (c == K_IGNORE);
+    event_disable_deferred();
+
+    if (c == K_EVENT) {
+      c = lastc;
+      event_process();
+      continue;
+    }
 
     /* Don't want K_CURSORHOLD for the second key, e.g., after CTRL-V. */
     did_cursorhold = TRUE;
@@ -943,10 +951,6 @@ doESCkey:
       did_cursorhold = TRUE;
       break;
 
-    case K_EVENT:
-      event_process();
-      break;
-
     case K_HOME:        /* <Home> */
     case K_KHOME:
     case K_S_HOME: