Fixed the bug that requesting protected or private action methods would cause 500 error instead of 404

fixes #3539

fixes #3431

allow using it from the gridview directly

Adds support for timestamps beyond year 2038.

InvalidCallException, InvalidParamException, UnknownMethodException are now extended from SPL BadMethodCallException

Adjustments code according to the standards.
Adding missing PHPDoc comment.

Fixes #3121: `yii\base\Application::bootstrap` may fail to load some components if they are specified as class names

Fixes #3091: Fixed inconsistent treatment of `Widget::run()` when a widget is used as a container and as a self-contained object

issue #3026

fixes #3034