Extract code for handling files into new library

[rosell-dk]

The library could be called "FileUtils".
Stuff that could be moved to file util lib:

• Helpers/FileExists (split it into HonestFileExists and FileExistsTryHard)
• DestinationPreparationTrait::checkDestinationWritable has some code for checking if a directory is writable
• There are also file functions in WebPExpress

Code that checks for sanity could go into own library, ie "SanityCheck":

• PathChecker.php
• There are also sanity checking functions in WebPExpress