e.g. UserEvent::trigger('event', 'class');
should result in a UserEvent to be passed to the handlers instead of
a base\Event.

And some docs punctuation.

After ensureBehaviors() call $this->_behaviors is an array already.
And since detachBehavior() unsets elements in $this->_behaviors, $this->_behaviors turns out to be an empty array after all iterations.

fixed #1021

fixes #1541

load extensions before preloading components to have aliases set for
extensions like debug module.

- allows to have dynamic definition of attributes depended on the
  instance
- there was no real need for it to be static. Places that used it static
  have been refactored.

fixes #1318