Add the ability to customize a class for rendering dropdowns in Nav

[PowerGamer1]

The yii\bootstrap\Nav should allow customizing the class used to render dropdowns (analogously how it is done in other places, for ex. yii\bootstrap\ActiveForm allows to customize the class for form inputs through ActiveForm::$fieldClass).

[PowerGamer1]

Added dropdown class customization also for yii\bootstrap\ButtonDropdown and yii\bootstrap\Tab classes in my pull request.