added functionality allowing to process all "text" fields in xml

This is a suggestion of how to get internationalization with firetongue to work with haxeui. The Idea is:

```haxe
    Toolkit.xmlTextTranslator = function(t:String) {firetongue.get(t);}
    Toolkit.init();
```

Now all xml that is loaded is checked for ".text" fields, which are than translated by the set function.

I do not know if I have choosen the best places to insert this, or function names. So feel free to reject this pull request.
But it would be nice if something like this could be integrated into haxeui ...