QmlDesigner.ItemLibrary: Start drag using QTimer::singleShot()

A drag might change the imports which invalidates the model exposed
to qml. Since QObjects are deleted when the model is reseted this did lead to
a crash in the qml engine, because the object emitting the signal was deleted.
To avoid this we call startDragAndDrop() asynchronously using QTimer::singleShot().
Now the object is always deleted after the signal is emitted not during the emit.

Task-number: QTCREATORBUG-11139

Change-Id: I92e7f9dfff33697b9ff0da148dc9da629723ab7e
Reviewed-by: Marco Bubke <marco.bubke@digia.com>