Updated to use mb_* functions

Length check.

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

issue #2852

- Gii diff markup is now copy paste friendly
- Removed StringHelper::diff()
- Moved phpspec/php-diff dependency from yiisoft/yii2 to yiisoft/yii2-gii

in case no dir is present

Moved file and path related methods from StringHelper to FileHelper, renamed StringHelper byte methods not to be misused as string methods

In contrast to php function basename() It that always uses \ and /
as directory separator.
It is needed for use on Classnames that contain namespaces.
Fixes #192

intregrated updates from version 1.1