Add a new option to configure the default drop action

Currently dolphin will always display a menu with three options for the current drop target, even if I never use links, or never want to copy anything.

This patch is by no means good: it feels more like a hack but I don't know how to do this in a better way: QDropEvent is used by KIO::DropAction and the menu is handled by KIO, not Dolphin.

So we fake a keyboard press to handle what we want, on the event. shift: copy alt: link control: move