fixed collecting values from tabs inside tabs

based on https://github.com/marianoguerra/json-edit/issues/30
it can be tested with this schema in demo - just try to validate
```json
{
  "je:hint": "tabs",
  "title": "App tabs",
  "order": [
    "ItemUpdated"
  ],
  "type": "object",
  "properties": {
    "ItemUpdated": {
      "type": "object",
      "je:hint": "tabs",
      "title": "Plugin",
      "order": [
        "DerivedAction"
      ],
      "properties": {
        "DerivedAction": {
          "type": "string",
          "title": "DerivedAction"
        }
      }
    }
  }
}
```