Cleaned up redundant code (and a typo missed earlier).

Removed redundant declarations and checks in the code.

Ensure previous CSS classes are not appended and reinitialized in the foreach loop.

Used ```Html::addCssClass``` and ```Html::removeCssClass``` more effectively.

Added closeButton parameter, assignment of CSS class corrected, and added php doc comments.

The previous alert widget within the advanced/basic app does not render multiple flash messages. The current implementation allows display of all flash messages assigned through setFlash. Also a validation is done for allowed bootstrap alert types.

Small edit for consistency with frontend config.

Small edit for consistency with /apps/advanced/backend/config/main.php. Sorry, my OCD kicked in.

Update script filter

```map``` and ```indexOf``` on arrays were only added in IE9.
Changed to make use of jQuery's equivalents.

fixes #1230

Duplicate

Duplicate APC requirement removed

issue #1228

* 'master' of github.com:yiisoft/yii2:
  Edited model.md up to Validation
  Just started editing

It is more obvious what to pass in trigger() method