Model->getActualFields() now can return array of field objects

Sometimes it's very handy to be able to get not only array of fieldnames and later do like this:
```
foreach ($m->getActualFields('editable') as $f) {
    $m->getElement($f)->sortable(true);
}
```
but to get array of field __objects__, like this:
```
foreach ($m->getActualFields('editable', true) as $f) {
    $f->sortable(true);
}
```

This saves some processing time and probably can be used in even nicer constructs, like (pseudo code):
```
$m->getActualFields('editable', true)
    ->each()
        ->sortable(true)
        ->display(array('form'=>'readonly'));
```