3.0 configuration improvement

With this approach it's also possible to configure Component, Behavoir & Helper in the app configuration. The configuration needs to look like the following:

```
'Component => [
    'Flash' => ['element' => 'Common.default']
]
```

Theoretically it's possible to configure also other things with it. If this isn't the goal, I can restrict it to Component, Behavior & Model.

Resolves https://github.com/cakephp/cakephp/issues/4296