Add event for coding post filter triggers

Trigger a new `filter.yiiGridView` event for yii grid view after grid filters are applied. This will enable developer to code a lot of use cases based on the grid filtering event. The filter `$form` can be accessed as a parameter to the event.
Created beforeFilter and afterFilter grid events

Events `beforeFilter.yiiGridView` and `afterFilter.yiiGridView` javascript events created for advanced use cases.
Updated comments for beforeFilter event
Updated comments for beforeFilter and afterFilter events
Corrected trigger element for beforeFilter
Simplified beforeFilter and afterFilter events

Removed `form` as a parameter to the events since it can be retrieved using `$grid.find('form.gridview-filter-form')`.
Simplified afterFilter event trigger code
Updated formatting - Created beforeFilter and afterFilter grid events
Updated comments for beforeFilter event
Code cleanup - beforeFilter and afterFilter events
Fixes #5117: Added `beforeFilter` and `afterFilter` JS events to `GridView`
Merge branch 'master' of git://github.com/yiisoft/yii2
Fixes #5126: Fixed text body and charset not being set for multipart mail
Fixes #5124: Added support to prevent duplicated form submission when using `ActiveForm`