ContentItem: Expose copy() method
fixes https://github.com/ubports/ubuntu-touch/issues/1766 fixes https://gitlab.com/ubports/core/content-hub/-/issues/3
Moving a file is not allowed due to app armor restriction on content_exchange policy. It fails when trying to delete the source file in HubIncoming.
More over using QFile::copy does not trigger the expected inotify event for media-scanner.
Provide a "copy" method and mark deprecated the "move" method.
Edited by Lionel Duboeuf