Fix drop menu position with urlnavigator drops

Commit 1e251d2f6a in kio broke drop menus when dropping on the URL
navigator (menus show up in the DolphinView rather than the URL bar).
This happens because in DolphinView::dropUrls() we set `this` as the
widget passed to KJobWidgets::setWindow() (in DragAndDropHelper::dropUrls()).

We need to replace `this` with the actual widget that received the QDropEvent
and that can mapToGlobal() the relative pos of the drop event.
Unfortunately this widget is not KUrlNavigator itself, but one of its
KUrlNavigatorButton children (private class, not exported). So
unfortunately we need a new API in KIO that exposes this child widget.

Differential Revision: https://phabricator.kde.org/D6684