Allow Drag and Drop between QSGItem

Implement a way to allow drag and drop between QSGItem. A use case would be to allow to connect two QSGItem by touching one and dragging it to the other. We should be able to take action on hover events and have access to the items where the drag started and ended.

If I'm not mistaken, in the previous QML MouseArea component would grab the mouse and then the other items would not receive any mouse events. Maybe it doesn't need dedicated classes, just letting the developper call ungrab() on the mousearea on exiting so that other items can grab it.