Make ActiveRecord::getNamespacedClass() static

For example:

```
public static function onAfterGroupSave()
{
     ...
}
```

Because i have no instance i cant use this helper method getNamespacedClass() inside this handler. But i want to do this way:

```
public static function onAfterGroupSave()
{
     static::getNamespacedClass('Category')->...;
}
```