Scope of ProcessedFile cannot be modified
A ProcessedFile is used to generate a preview or crop-scaled version of a File. Actually a Task implementing the TaskInterface defines the basics and an accordant Helper object takes care of the modifications to that file. There are pre-process and post-process signals that seem to be useless in modifying the scope of a particular task, however.
Imagine a file shall be pre-processed (e.g. color profile transformation) and that result shall be used for resizing.
The current task implementation is using the original file directly, however the task actually shall define which is the source (the original file) the modifications shall be based on.
This change modifies the TaskInterface with a getSourceFile() method to overcome this limitation.