New truncate and truncateWords methods

New truncate and truncateWords methods, https://github.com/yiisoft/yii2/issues/2941
Update BaseStringHelper.php

Length check.
Update CHANGELOG.md
Update BaseStringHelper.php

Updated to use mb_* functions
Test case for new truncate methods.

Test case for new truncate methods.
Update BaseStringHelper.php
Issue #3154 - DataColumn filter model validation error indication added
Issue #3154 - GridView - list of filter model validation errors added to summary
Issue #3154 - DataColumn checkForFilterErrors() now use Html::addCssClass
Update BaseStringHelper.php
Changelog update
made grid\DataColumn::getDataCellValue() public

allow using it from the gridview directly
BaseListView - errors rendering added
GridView - error rendering scheme changed
ensure gridview query is reusable

apply pagination and sorting to a clone of it.
GridView/DataColumn - moved error list display into GridView
Fixes #3036: Upgraded Twitter Bootstrap to 3.1.x
DataColumn - error detection moved into renderFilterCellContent()
Fixes #3212: removed unnecessary prepare() call.
install bootstrap 3.1 by default but allow 3.0.x