Add StringDictProperty

Ensures that all dict values are string types. The DictProperty
converts the value '802.11' (see example below) into a Decimal
property. The StringDictProperty prevents this conversion.

Example:

```javascript
{
  "name" : {
    "de" : "Stadt",
    "en" : "City",
    "xx" : "802.11"
  }
}
```