Inject node - let once delay be editable (#1541)

* inject once with delay

* test for inject delay at once works

* give access to the once delay of the inject node

* change event not needed in HTML

* code review with Dave

* rename test

* tests for default and optional delay

* test once with delay and repeat

nodes/core/locales/en-US/messages.json

@@ -59,7 +59,8 @@
             "Sunday"
         ],
         "on": "on",
-        "onstart": "Inject once at start?",
+        "onstart": "Inject once after",
+        "onceDelay": "seconds, then",
         "tip": "<b>Note:</b> \"interval between times\" and \"at a specific time\" will use cron.<br/>See info box for details.",
         "success": "Successfully injected: __label__",
         "errors": {