DropJob: Track supportedActions in dropEvent before showing menu
The source of a drag will provide possible actions allowed for the drag. One action will then be selected by layers below KIO.
When performing a drop on a folder KIO will prompt the user as to which action should be performed. This fails to take into account what the source allows.
This is intended as a bugfix in order to support a plasma-desktop: plasma/plasma-desktop!2662 but is semantically standalone.