Fixes #3240: Added support for assigning an anonymous function to `yii\widgets\ActiveForm::fieldConfig`

Better fix for #3443 that does not break BC and allows widget to be extensible.

Fixes #3443: Fixed `yii\bootstrap\Nav` and `yii\bootstrap\Dropdown` were generating wrong ids for submenus

Incorrect character for generating docs. See http://www.yiiframework.com/doc-2.0/yii-bootstrap-activeform.html

The registerPlugin is not required.

```
vendor/bin/indent --spaces -r framework/
vendor/bin/indent --tabs -r . --pattern=*.json
vendor/bin/indent --spaces -r . --pattern=*.md
vendor/bin/indent --spaces -r . --pattern=yii.*.js
vendor/bin/indent --spaces -r . --pattern=*.css
```

This change fixes the issue #3292. The details were mentioned in the comments to the issue.