dropjob: add DropJobFlag::ExcludePluginsActions flag support
This commit corrects previous one: 71b2b38b which incorrectly recognized drop action from Places View (based only on missing Qt::MoveAction which had side effects described in BUG 514697)
This time DropJobPrivate class supports additional DropJobFlag::ExcludePluginsActions flag which indicates that potentially destructive Move-like plugins actions should not be placed in popup drop menu.