[FileCopyJob] Clean up after cancelation of file copy operation

Summary:
Delete the destination file if the copy job was canceled. In case Overwrite flag
was provided to the job, delete the .part file.

Test Plan: Unit testing and manual testing done.

Reviewers: dfaure, dmitrio, ngraham

Reviewed By: dfaure, ngraham

Subscribers: kde-frameworks-devel, ngraham

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D18904