Fix the initialization sequence of the `adaptee` train in `Adapter`.

`adaptee` is a mandatory trait in `Adapter`, and it was initialized
like this:

```
         super(Adapter, self).__init__(**traits)
         self.adaptee = adaptee
```

The problem with this is that properties that depend on `adaptee` may be
triggered in the super-constructor, when `adaptee` is not yet defined.

The new solution:

```
         traits['adaptee'] = adaptee
         super(Adapter, self).__init__(**traits)
```

fixes that.

Note that this solution is better than:

```
         traits.adaptee = adaptee
         super(Adapter, self).__init__(**traits)
```

since in this case the trait change notification for `adaptee` is not
triggered.