add possibility to use a callback function as naming attribute for images

[ckilb]

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[ckilb]

This pull requests makes it possible to use a callback function for the naming option for images ("keep" vs callback function vs anything else for random).
For this I've moved the logic for generating random file names from the Multup class into the File.php class. I think it would be a good idea to get completely rid of the Multup class :) Like you said in one of the commits, it's a disaster ;)
I wasn't sure if the file name generation still should belong to the File Field class. But I think it's still fine to have this being part of the responsibility for this class.