NavBar class not easily extendable due to html in the init function

[yii-bot]

This issue has originally been reported by @kanyuga at yiisoft/yii2#6689.
Moved here by @cebe.

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The init function of the \yii\bootstrap\NavBar class has several lines of code that output html. Since every widget class should contain a call to parent::init(), this class cannot be easily extended since the html in the init function will also sent to the output. I encountered this problem while trying to extend the NavBar class to remove the div that wraps the individual menus.

A proposed solution is to move the bulk of the code in the init function into a separate function.

[cebe]

same improvement as applied for yii core classes should be applied in bootstrap, see yiisoft/yii2#11915 (comment)

[razvanphp]

I would like to fix this, but due to the nature of the Widget, it needs to echo in the init, to be able to include other menu items in between begin declaration and end.

Would just moving the big echo chunk out of init into a private function work too?