realpath(false) = current working directory.
This can cause problems with getAlias() which returns false.

see yiisoft/yii#3113

- added example code for application components
- added missing short description

[ci skip]

- rename FileHelper::mkdir for API consistency
- changed default permission for directories to 775 instead of 777
- added some properties to classes that deal with files to allow control
  over directory permissions.

I18N WIP.