prevent call strlen function twice

fixes #1293

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

used e.g. for less client side compiling

"Item" parameter keys are the values of the checkbox list,
while the values are the checkbox labels.

code style section has been added here:
https://github.com/yiisoft/yii2/wiki/Core-framework-code-style#self-vs-static

verification code for properties:
https://gist.github.com/cebe/7566916

defaults overided settings

It was mentioned in AssetManager::publish phpdoc.