Support "uniqueItems" in configuration UI

[letmaik]

     "myext.option": {
          "type": "array",
          "uniqueItems": true,
          "items": {
            "type": "string",
            "enum": [
              "A",
              "B"
            ]
          }
      }

A value of "myext.option": ["A", "A"] shows up as error ("Array as duplicate items") in the JSON view but not in the UI view. Both views should have consistent errors.