Fix wallpaper destination path
Summary:
- Correct the destination path when downloading images.
- Switch from FileCopyJob to CopyJob to allow showing file overwrite dialog.
BUG: 379469
Test Plan: Drop files from Firefox to the desktop to set the wallpaper successfully.
Reviewers: Plasma, mart, ngraham
Reviewed By: Plasma, mart, ngraham
Subscribers: mart, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D26525