FileSystem: Add method makeWritable()

[janbarasek]

• new feature
• BC break? no

Very often I need to write to a directory where I cannot guarantee the correctly set permissions in advance.

This feature can try to fix the permissions according to the settings, or throw an exception.

Directory permissions are set recursively.

[JanTvrdik]

1. fix permission is bad name; it should be called changePermissions (with removal of default values for $dirMode and $fileMode) or makeWritable (and keep the default values)
2. i'm not sure if you need the $recursive parameter as everything in FileSytem is always recursive

[janbarasek]

@JanTvrdik Fixed, thanks.