QmlDesigner.ItemLibrary: Fix Drag Drop on Mac OS X

Since grabMouse() is limited on Mac OS X, we cannot use
the custom drag and drop code anymore.
The icon stays visible.

Task-number: QTBUG-34201
Task-number: QTCREATORBUG-10414
Change-Id: I92009c34881d086049e50b0c9ef109a7149ac657
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>