Fixes #5494: Added support for specifying a menu header as a configuration array in `yii\bootstrap\Dropdown`

Fixes #5799: `yii\bootstrap\ButtonGroup::buttons` can take all options that are supported by `yii\bootstrap\Button`

Pass all the button's config array to Button::widget() instead of creating new one item by item. Now allowed to pass in ButtonGroup button's configs any possible attributes(like tagName, encodeLabel, etc).

Fixes #5735: Added `yii\bootstrap\Tabs::renderTabContent` to support manually rendering tab contents

Fixes #5708

Fixes #5570: `yii\bootstrap\Tabs` would throw an exception if `content` is not set for one of its `items`
Fixes #5572.

close #5355

Fixes #5232 and #5231

as discussed before, better do this change before RC to avoid breaking BC in GA.

Fixes #5143: yii\bootstrap\ActiveField::label() should not encode label if it is explicitly specified. [skip ci]

- in composer the `~` operator is just a shortcut for `~1.0` = `>=1.0.0,<2.0.0`
  it does not prevent dev versions to be installed but neither does `1.*`
- dev versions are constrolled via `minimum-stability` setting and the
  `@dev`, `@beta`, `@stable`,... settings.
- setting bower packages that ship compiled files only with the stable
  release to use `@stable` in composer.json of the applictaions. This
  setting can be removed when we remove the `minimum-stability=dev`
  setting after GA.