Fixes #184: `yii\bootstrap\ButtonDropdown` widget did not use the correct JS plugin, was `button`, changed to `dropdown`

Fixes #107: Added `yii\bootstrap\Collapse::$autoCloseItems` to allow keeping multiple items open at the same time

Fixes #157: Active status of multilevel submenu items of the `yii\bootstrap\Nav` class is determined correctly now

close #187

close #131

Fixes #196: Remove `role="form"` from `yii\bootstrap\ActiveForm` according to new aria specification

Fixes #174: Added `yii\bootstrap\Tabs::renderPanes()` to allow extending the class to manipulate the content between the tabs and the content

Fixes #162: Fixed `yii\bootstrap\Nav` not taking explicit `active` into account when `activateItems` is off

avoid unbound constraints

https://github.com/yiisoft/yii2/issues/12154

Two changes:
1. Documented previously undocumented 'encode' item option.
2. Moved `is_string($item)` check to occur BEFORE `$item` is handled as array. When `$item` is a string `$item['visible']` would not be an error and will return the first character of that string. Doing `!$item['visible']` on the first character of a string does not make any sense.

Fixes #145: Added the ability to customize the class used to draw dropdowns in `yii\bootstrap\Nav`, `yii\bootstrapButtonDropdown` and `yii\bootstrap\Tab` widgets

Allow URLs to be used as Dropdown items in Tab Widgets #113

Changed error message for content/url option in Tab Widget Dropdown items to be more specific based on feedback by @cebe

See #113

See #113

Fixes #113

grammatical adjustment